You can use a number of different kinds of shells.
First, actual Neothauma shells of the type the shellies would live in in Lake Tang. However, these are difficult to get in large numbers and often quite expensive; Aquabid's the best bet for acquiring them.
Turbo shells, which are the shells of marine inverts, are available at pet stores, usually meant for hermit crabs. These can be pretty but rarely match, are too heavy for the shellies to move, and again are difficult to buy in large numbers.
Craft stores often carry shells of assorted types - turbos with odd coatings, oddly-shaped shells, and so on. The only thing to watch for with craft stores shells is to choose based on the fish. Round shells with round openings, as light as possible, and about one to two inches in diameter are the best. Craft store shells can vary from surprisingly cheap to somewhat overpriced and deserve a good lookabout.
Escargot shells are cheap, light, the correct shape and size, and fairly natural-looking. But they can be difficult to find outside of fairly populous areas. Gourmet stores, upscale groceries, and French restaurants are the safest bet. Escargot shells do come in different sizes but the most often available size, Extra-Large, is just about perfect.
Sometimes the shells are packaged with canned escargot but I know a lovely recipe if you have extra!
Non-shell options:
PVC elbows, as you mentioned, are common for breeding and rearing tanks. Generally there's a cap on one end although that's optional. They're easy to care for, cheap, and it's extremely easy to remove the shellies from them, but of course they're not at all natural and one misses out on nearly all of the natural behaviors of the fish.
Small terracotta pots with holes cut in the side are also a possibility, but this completely erases the natural behaviors. This essentially creates cave-dwelling fish, when most of the fun of keeping shelldwellers is the shelldwelling part! It's a possibility, but has few if any pros and a lot of cons: fewer pots will fit than shells, the fish can't spawn in the traditional way, they're far from natural, they easily accumulate algae because of their porosity, and so on.
Escargot shells (note the round opening):
