Stop by any of the big local fish stores in the bay area. They have empty shells. Ask them if you can buy shells. The cool ones will just give you the empty shells for free. As for snails dying, 30% of my snails mysteriously die a couple months after adding them. Theres algae. No predators. They just die. I add new snails probably 3 times a year. Have you thought about target feeding your hermits/shrimp with a turkey baster and some frozen food?