If you keep shell dwellers and you’re having trouble getting shells or getting your shellies to use them, an alternative is PVC. Use small elbows with caps. One advantage to using PVC elbows as a shell substitute is that it is more difficult for the occupant to get “lodged” in the elbow and not get out. Another advantage is that you can remove the cap on the end of the elbow to extract the PVC, the fish, or fish’s eggs separately if you need to for some reason.
Note that there is no guarantee that your shell dwellers will use the PVC to breed (or use them at all). If you’ve kept shellies long enough, you know that some species can be very picky about not only the type of shell but even specific shells in the tank. I’ve used many types of shells in the past for a variety of species, and my experience is that no one species gravitates towards one type of shell. Because not all shell dwellers are equal (i.e., size, temperament), you might have to experiment with different PVC sizes to see what your shellies prefer. Just like with actual shells, it might be a bit of a crapshoot.
A shout out to fellow Tanganyika shell dweller enthusiast Ryan Garland, for the topic of this post. He may not have originated the idea, but he is who I got it from.