the best thing to do is phone around pet stores that cater for tropical & marine pets or lizards and snakes ect as most general pet stores only cater for common animals like furry critters or dogs / cats ect

or if you live near by a beach or seaside
you can try hunting for shells

if you find any shells from the wild you must
1. obviosly make sure there arnt any residents present lol
2. wash the shells to remove sand and debris (dont use cleaners or bleach)
only water.
3. pop the shells in boiling water for about 1hr to kill
any bacteria

try to find shells about the same size as you have or slighty bigger like a size or so more as its like shopping for shoes lol

dont worry too much if they dont change shells often as they are very picky
and will only live in one they feel happy in
but after moulting they do need a size bigger
but try to have atleast 2 extra shells of various sizes per hermit crab

alternitavly there are online hermit crab stores that sell shells if you get stuck hunting localy for them

im currently building a website to cater for hermit crabs
which can give you better info and links resorces
check it out here

hermit land

its still under construction so most links go back to my home page


anyways
hope that was of some help

thanks
pugz