NEW YORK -- Walking through the doors of Mr. Throwback is like climbing through a wardrobe that leads you to the gates of sports Narnia.

The vintage sportswear store in the East Village of New York City is a sea of snapback hats, jerseys, Starter jackets and 1990s toys. Michael Jordan posters are plastered everywhere. An oversized Knicks watch hangs on the wall. There's a phone that sings the Nickelodeon theme song and a Sega Genesis, unopened and untouched in its box like a time capsule.

"This store," says Mr. Throwback employee Tony Messina, "is like if my childhood puked all over the walls."

Sports fans like Messina, nostalgic for their own childhood remembrances, have turned Mr. Throwback into an institution. The COVID-19 pandemic -- which put live games on pause -- poured gasoline on an already booming interest in retro sports apparel. Without live sports to watch, fans looked to the past -- epitomized by the smash hit docuseries "The Last Dance," which averaged more than 5.6 million live viewers every week and was ESPN's most watched documentary of all time. That interest also led to increased sales of sports cards, merchandise -- and throwback gear.

Hats that sold for $60 a half-dozen years ago are now selling for more than twice that. (When Phoenix Suns guard Devin Booker recently wore a retro USA Basketball hat, the market price jumped from $30 to $150.) Jerseys that once went for $150 now cost $400.

"The same way that sports cards boomed during the pandemic," says Mr. Throwback owner Mike Spitz, "so did sports retro gear. My competition isn't just eBay anymore."

Now, luxury brands like Gucci, Celine, Versace and Prada have leaned into '90s sportswear in their 2021 spring and summer collections. Etsy, eBay and national retailers like Urban Outfitters have started to sell retro sports merchandise as well. Even Vogue got on board.

Mr. Throwback, though, is one of the few local stores in the United States that specializes in the niche. There are others -- like Throwbacks Northwest in Seattle, as well as And Still in Los Angeles, which is now online only. But Spitz's store has become the go-to place for athletes, celebrities and sports fans from across the country seeking rare, original throwbacks -- genuine vintage, not replicas.

"I've had people go to the NBA store and ask where the throwback jerseys are that aren't Mitchell & Ness," Spitz says. "Even employees there tell them to go to Mr. Throwback."

Athletes often come looking for retro merchandise for their alma maters or favorite retired players. NBA fashion star (and Miami Heat forward) P.J. Tucker recently wore an old-school Texas Longhorns jacket he bought from Mr. Throwback, which led to an influx of inquiries for UT gear. Way back during the peak of Knicks Linsanity, Jeremy Lin came by, and other hoopsters like Andre Drummond, Chris Douglas-Roberts and Iman Shumpert (and his singer-songwriter wife, Teyana Taylor) have hopped in since. Most recently, Los Angeles Angels infielder and New York native/former Yankee Andrew Velazquez stopped by to browse with his girlfriend.

HOOK 'EM. P.J. Tucker in his "Texas Longhorns" Air Jordan 1 customs by @SHOECHEF_. pic.twitter.com/gNlLPOe4XO

— Complex Sneakers (@ComplexSneakers) January 14, 2021

Spitz, 39, looks like the type of guy you'd imagine regularly calling into New York sports talk radio. On a Friday afternoon, he sports an oversized Patrick Ewing jersey and a snapback hat featuring the store's logo while roaming up and down the aisles, sifting through his inventory with the focus of Indiana Jones brushing off Egyptian ruins.

When it comes to growing that inventory, Spitz takes a multipronged approach. He browses through more than 100 Facebook groups for clothing, digs through Grailed, Mercari and eBay, buys sports stock from other non-sports-specific thrift stores and attends vintage clothing conventions. In Mr. Throwback's storage room, more retro merchandise awaits, ready to swap in and out as needed.

To keep the shop buzzing with athletes and fans alike, Spitz closely follows trends in the sports world -- and documents them on the store's growing Instagram account. If the Syracuse hoops team starts performing well, up goes the demand for Orange basketball gear. When a 30-for-30 documentary aired on the University of Miami, Hurricanes threads got hot. "The Last Dance" dramatically increased demand for retro Bulls merchandise. This is especially fun for Spitz when the Knicks play well, because he can bring out the backlog of gear that he has collected over three decades of Knicks fandom.

And whenever he goes to Madison Square Garden, he searches through the pile and picks something special for himself -- often wearing it with the tags still on. It's when he's most reminded of why he opened the store.

"Wearing my gear to MSG," he says, "makes me feel like a kid again."

Spitz got the idea for opening a throwback store in 2012, when he was living with his parents on Long Island. At the time, Spitz made his money at an accounting job he hated while also working part time at a children's gym. His parents pushed him toward the stable path, to work a classic 9-to-5 job and collect a check, but Spitz kept feeling an itch for something more.

"I was going through a quarter-life crisis," Spitz says.

He started spending time at Dekalb Market in Brooklyn, and soon quit his day job and opened a stand of his own there. He paid $500 a week in rent for an 80 square-foot space, working Tuesday through Saturday with makeshift inventory. He started by buying old jackets and jerseys from friends, perusing the aisles of TJ Maxx and Marshalls to find good deals he could repurpose, and browsing other sources like eBay and consignment shops.

"I just loved it so much," Spitz says. He realized: "This is it. This is the rest of my life. This is what I want to do."

That hustle lasted for eight months before new landlords shut down the market in November 2012 to build a high-rise building. Unclear on what his next step was, Spitz did what he always does: He went thrifting. Retail therapy led him on a shopping spree in the East Village, where later that same month he stumbled upon a storefront, set up with clothing racks.

He contacted the landlord, who told him the space was his for $5,700 a month. Painting the shop to match his vibe would cost another $200.

He had $6,000 total in his bank account.

"I put down everything," Spitz says. "I had nothing left. I was like, 'I'm doing this. F--- it.'"

Mr. Throwback soon started catching on in the underground New York fashion scene. The East Village storefront opened in November 2012, and two years later, Mr. Throwback received a mini profile in a New York publication called Fat Man Magazine -- an issue that happened to feature musician Kid Cudi on the cover. A few days after the story was published, Spitz watched the door swing open.

It was Cudi, looking for Cleveland sports gear.

"Hi, I'm Scott," Cudi said.

Spitz stood still, jaw agape.

"I couldn't even acknowledge him," Spitz says. "I was so starstruck."

Since then, the shop was named in New York Magazine as the city's best nostalgia shop. Fashion designer Alexander Wang has sewn sweaters in the store. Rapper DaBaby wore a Larry Johnson Charlotte Hornets jersey from Mr. Throwback when he performed on Saturday Night Live in 2019. Spitz once received a call from comedian Pete Davidson, asking him to open up on a Sunday so he could shop with his then-fiancé, pop icon Ariana Grande, who sang her own music while flipping through Starter jackets and T-shirts. Other celebrity shoppers include rapper Fabulous and actors Susan Sarandon and Scarlett Johansson.

On a recent Friday afternoon, Nate Reznicek, a 29-year-old from Burlington, Kansas, who works as a data analyst in New York, purchased a retro Kansas City Chiefs Joe Montana jersey and a Kansas Jayhawks Starter jacket.

"If it's lasted this long, it's gonna stay cool," Reznicek says. "If someone thought it was worth keeping in good shape for 30 years, it's unique."

As Reznicek surveyed the store for gear, Messina and Spitz led him along like museum tour guides, giving a sense of each item's history. Messina knows what it's like to be on the other side as a former regular customer. Before he started working in the store, he stopped by a couple of times a week to buy clothing and chat with Spitz. Spitz, impressed by Messina's encyclopedic knowledge of the merchandise, asked him if he wanted a job.

"I walk around, I'm literally in my childhood dream," Messina says. "That's exactly the point of the store."