Rubber stamp store. Just shelf after shelf of rubber stamps and rubber stamp accessories.

I came across a very narrow, 5 story store in Japan, a bit off the beaten path in Akihabara, selling exclusively N scale model trains. They had a floor or two dedicated just to the models, another one for scenery, and at least one just for repair parts for your trains. It was really cool to get to see a store that was pretty clearly a passion project for the owner(s) apparently doing well, and filling quite the niche in the hobby (I imagine)

I think my favorite is Nothing Bundt Cakes that sells Bundt cakes. It's a chain, and yes I only like it because of the pun.

An entire store full of african-american romance novels. In a VERY white area. It did not stay open long 😞

My town has a mushroom shop.

Those stores that only sell Rubber Duckies come to mind.

In Turkey we have shops that only sell pickles!

It's very recently closed its doors, sadly, but you'd be hard pressed to find a more specific shop than long-time Edinburgh landmark, Borland's:

I used to live across the street from a ballbearing shop.

spirit halloween seems just the place for a certain one night event.

Cycling clothing. No bikes, no helmets, just more lycra than a gimp convention.

I had to go to a store that only sells custom made stirrups as my kid needed some for their horse. I was absolutely overwhelmed about how many differences there are about them. Kid was happy.

There used to be a store in my town called “Just Socks” that sold, just socks

There's a bunch of oddball stores in the Pike Place Market in Seattle but the Left Hand store was the most specific place. All left hand tools and stuff. Lots of different scissors.

Batteries + Bulbs.

They sold nothing but batteries and lightbulbs.

I've been in a Christmas store that all year long only sold Christmas stuff.

Art Smart's Dart Mart and Juggling Emporium in Milwaukee.

I have been in a shop only for set equipment for cooking book pictures

I have not, myself, been there, but San Francisco has the Pirate Supply Store.

https://en.wikipedia.org/wiki/826_Valencia

826 Valencia is a non-profit organization in the Mission District of San Francisco, California, United States, dedicated to helping children and young adults develop writing skills and to helping teachers inspire their students to write.

826 Valencia runs San Francisco's "only independent pirate supply store". The store is the front entrance of the tutoring center and has the look and feel of an authentic pirate shop.[5] It sells pirate clothing, eyepatches, compasses, spyglasses, pirate dice, skull flags, and secret treasures. It features handmade signs, scattered around the store, offering tongue-in-cheek wisdom, such as "Uses for Lard" (#5: "Lard Fights") and "Guidelines for New Shipmates" (#4: "No forgetting to swab"). Unsuspecting visitors are sometimes treated to surprise "moppings."

Shoppers can also find back issues of McSweeney's Quarterly Concern, books published by McSweeney's, and literary compilations written by 826 Valencia students. The books of student writing published by 826 Valencia include forewords, illustrations and interviews from public figures such as Former San Francisco mayor Gavin Newsom and actor Robin Williams.

The Pirate Supply Store was originally created through necessity. After being turned down for regular use of church basements and school facilities, the founders discovered an empty store on Valencia Street. City ordinances required businesses in that area of the city to be either retail or catering, so the Pirate Supply Store was opened in 2002 as the "legitimate" business front for the writing center. It also helps fund the tutoring program.[6]

I have seen lots of niche stores, but what usually gets me is that they're in very premium locations. How is that empty Custom Umbrella Emporium still open while the packed coffee shop next door can't afford their lease?

I went into a store that specialized in selling canes when I visited London.

There's a chain of shops in my country that, at least for a while, pretty much only made and sold bean bags. Freaking fantastic ones, too.

There was a store in Vegas that was magnets. Fridge, city, anything magnets.
And sardine stores in Portugal, airports and elsewhere.

There's a "craft"-art shop in the same cave I work in that exclusively sells racism.

Synthesizer store. RIP to Seattle's Patchwerks. Such a damn shame they went out of business.

In County Cork in Ireland I once saw a shop that was a "Gentleman's hairdresser and railway bookshop."

Disc Golf Mart in Columbus, Ohio.

There’s a small town I drive through frequently that, for many years, had a Christmas shop. It was always open, usually empty. It closed down last year.

My favorite was Wizard's welding and hair salon, sadly closed now.

The Catholic gift shop near me that's open like 8 hours a week. Maybe it's money laundering.

Envios a Cuba, some shops here just send items to Cuba for you. Only to Cuba, only from here

Mixed tape store. This was like 12 years ago, long after streaming was a thing. A buddy of mine went in and asked if they sold CDs and the guy legit said, "No. Just mixed tapes." Pretty sure it was a front for something.

I’ve been in a knitting store before. Just yarn and needles and patterns. I can’t imagine how they stay in business.

Saw a store that was just rubber ducks. They had celebrity ducks and colored ducks and all sorts of similar things.

Probably Pop Mart. A three-story shop filled with nothing but these odd little collectible toys of cartoon characters. It was absolutely mobbed.

Magic stores.

San Francisco has two of them: Misdirections, a store that's been there for decades, and Houdini's/Vanishing Inc. in touristy Pier 39 who have a second location near Disneyland.

Also, the San Francisco Archery Shop seems very specialized in this age of big box sporting goods stores, but again they've been around for decades it feels like.

The Bee Healthy Honey Shop in Oakland, selling honey, beeswax, and related products and also beekeeping equipment. It's run by a Yemeni family of beekeepers.

Stores that sell clothes branded with one college's logo / name.

japan. anywhere in japan

I've been in not one, but three different round the year Christmas stuff stores.

There is a chain of stores in Europe only selling Gouda cheese (Henri Willig).

In Hamburg I've been in a shop specialized on umbrellas.

In Rotterdam I've found a shop selling only socks.

The Glow In the Dark Cat Shop

You'll never guess what it sells

https://shop-york.co.uk/places/cats/

Kite store in Chicago (it's really not that windy there, really)

A glove shop in Lisbon.

there's a store in seattle called The Purple Store that only sells purple stuff!!!

I just bumped into another oddly specific store: Darts Central in SF

Tennis rackets and Kites

https://youtu.be/BfnfOdQVHAQ

90’s throwback. Of note - Stacks of Slax and the Bolt Bloke.

Do the running stores that are popping up everywhere count