You can buy the hard cases at Dicks Sporting Goods, Gander Mountain, Bass Proshops, and those types of places. We have some of these, but prefer the "soft" kinds.
If you google waterproof wallet, you'll find several sites. We carry a little flexible plastic pouch that easily fits in the pocket of our swim shorts/trunks and anchors to the key hook of our shorts. (We buy water shorts at Lands End just because they fit our "port" lifestyle.) This way, it feels like a wallet and works great!