I haven't kept marine, but I know they do grow and I've seen empty shells for sale for them at LFSs. Just buy a few bigger shells and scatter them around the tank.
Do most LFS's sell natural looking shells? I hate those tacky painted crap shells. Ive seen land hermit crab shells at LFS's but I dont think these are ok for marine hermits.
The ones I've seen were for marine use. Don't get the land hermit crab shells; I wouldn't want you to add any potential paint contaminent to your tank.
I wouldnt get anything painted, but as long as its a natural shell they'll be fine with it. I've actually seen hermits dragging around a piece of live rock as a shell
I went and collected a bunch of shells of all different sizes from rock pools and scattered them in my tank. The hermits ended up killing each other the few remaining killed my snails and took their shells. In the end it was a waste of time really . Make sure if you collect your own shells that they are hermit free as well!
You can also buy empty shells at craft stores, they often come in bags with different sized shells in them. Just make sure the shells aren't painted and you'll be fine....