I ordered these from Amazon and they have a good variety of small to medium sized openings. Both of my crabs have switched into one or more shells from that lot.

Two of the shells have decorative cutouts, which I didn't realize before I bought them (You can clearly see them in the photo, though). I didn't put those in the tank because they can cause injury to the crabs.

I also just ordered about 8 shells from Richards, which someone else mentioned in an earlier comment. They haven't arrived yet, but they have a HUGE selection with a range of prices.