im in the same boat my self

my hermies dont seem to interested in the ones we got from the pet shop

i tryed my local beach and found every shell exept the ones you need lol
i did find some nice pebbles and stuff though

im off this week sometime hopfully to the reptile and aquatic store in hope they have some as regular pet stores dont cater for these and if they do have shells its mostly not the type needed

there are plenty online shops that sell them and consider that as a last resort but stay clear of painted or hand made plastic shells

athough they look cool they probably contain metal in the paint or are toxic

ill dig out some online stores that cater for hermit crabs and sell shells ect
ill post em here if i find any good ones

but i recon the best place to try would be aquarium and lizard and tropical pet shops

thanks
pugz