Rather than specialized, I enjoy stores that sell odd mixes of products. In Des Moines – not a very large city – we have a new place that sells African groceries and used appliances.
There’s a store near my home that’s called Bulldog Battery.
If they don’t have it in stock, they’ll BUILD it!
In Washington DC, there’s a MAP STORE. Maps from all over the world.
I’ve seen a store that just sold magnets. I have no idea if it’s still in business, but it would be the place to go for all your magnet needs.
I forgot The Umbrella Shop!
Vancouver also has The Flag Shop with two stores there and more across Canada. There’s a kite store as well. Or you could go out to New Westminster and visit a store that only has electric razors, replacement parts, travel cases, and so on.
In mythbusters, they featured an used exhaust pipe shop. Selling nothing but used exhaust pipes.