Well, you could buy/trade shells with a nearby crabber, or usually pet stores will let you buy the empty shells in the tank with the crabs, but these are often painted or inappropriate for crabs.
I usually get my hand me downs from newbies, I give fresh shells to a newbie on the way home or during quarentine, then I have his empty shell to offer to mine, and when he moves into the main tank he usually changes again, and everyone goes nuts for his shell, leaving THEIR shell empty, and someone wants that one. This cycle can go on and on and on, leaving almost every shell worn once or twice, so it's now a 'used' shell

The jumbo I just adopted was in a way too small shell, so he changed four times before choosing a bigger one! So I now have his old shell, and all the one's he tried on but didn't keep that will smell 'used' to my jumbos that are underground molting

In the wild, when a large crab finds a new empty shell, a smaller crab will wait beside him, and a smaller one will wait behind him, so they all get the hand me down thats a bit bigger then theirs, they can have huge 'shell swap chains' sometimes, the big guy changes, and the smaller guy takes his old shell, and so on.
Hope that helped
