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

Screws. A whole shop especially for screws and nuts, and you could buy bigger boxes, or smaller boxes, or only one or two, and they took their time to give advice, always with the same friendliness and helpfulness, whether you left a few cents or some real money.

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)

Tennis rackets and Kites

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.

In Turkey we have shops that only sell pickles!

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

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

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

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

Batteries + Bulbs.

They sold nothing but batteries and lightbulbs.

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

My town has a mushroom shop.

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.

Those stores that only sell Rubber Duckies come to mind.

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.

It’s not super specific, and makes sense for where I saw it, but I’ll never forget the Wisconsin tourist trap I drove past as a kid that has a big sign out front advertising “Cameras, Cheese and Guns”.

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?

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

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.

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

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

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

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

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.

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

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]

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.

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

Disc Golf Mart in Columbus, Ohio.

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