You can buy shells online. Or sometimes you can find them for decorations in places. But they wont like them if they are not lined with mother of pearl (that smooth shiny feeling on the inside of most shells).
Our crabs didnt make it because they were "intertidal" crabs and we were not able to recreat the ocean waves and such (stupid us brought them home from the gulf of mexico).
but i did a lot of research.
You also need to pay attention to what shell they are in (snail shell, or the more scalloped type one ect). reason I say that is because from what I have read they can only go into a LIKE shell , because thier adomens FORM shape from the first shell they ever crawl into. So if they go into a rounded shell, then they are stuck with rounded shells for life.
But they are a large assortment of crab shells availible. There is a hermit crab forum I once visited where the people are extrememly nice and informative.
...if you are intrested in the name, pm me, and I'll give it to you. They often trade shells with one another, and are very very helpful.
(hope I didnt do a no no by posting that).