Check out cichlidbreeding.com for shells. The prices are about as good as you'll find anywhere, and they do send extra's. When I was setting up the tank for my multi's I ordered 40 gold-mouth turbo shells, and received 45.
Sometimes you can find good deals on aquabid.com.
Othersise asking someone at a LFS mught be useful too especially if they deal in saltwater. Otherwise I'd goto CostPlus and buy the shells they use for escargot, it's like $6 for canister of 25shells.
If you are still looking for shells, feel free to contact me. I used to sell hermit crab supplies, and have decided to stop selling shells, so I have thousands available, and can sell any quantity at very good prices.