When looking at blanks it's important to consider the type and hardness of the metal. Hard steel can be difficult to get a good clear stamp on and can potentially damage your stamps. Sterling silver is an excellent choice because it's affordable and soft enough to allow you to make some pretty cool stuff.
You can buy blanks on-line or at your local craft store. The good thing about buying them from a brick and mortar store is that you can talk to the person you're buying them from and make sure they'll meet your needs. If you build a relationship with people at the store they'll keep a look-out for good deals and let you know when they see cool stuff.
But really you're only limited by your imagination. So go out there, grab some blanks, and get stamping!!!