Any Walmart or similar should have shower caddies. With a 10 gallon, you are best getting the small ones. A craft store (like Michael's) will have nets and shells. I found that pet stores (Petco or Petsmart) will have a few items (dishes, Ecco Earth, climbing vines) but they usually sell the 'Hermie' stuff that is not recommended because of the chemicals.
There are a couple of one line sites that you won't go wrong with.
Hermit Crab Addiction - Welcome To The Hermit Crab Addiction
thecrabbagepatch.com
Welcome to El*s Shells!, For all of your hermit crab needs...
The Happy Hermit Crab - Back to the Kitchen