Weber doesn't sell the tanks anymore, and they are only to be used for 10 or so years anyway. I've heard that RV dealerships have them, but when I've called around they were more than 100 bucks. True Value is a good option, too, and I think Lowes and Amazon have them as well. I finally got super-lucky when I was visiting a Weber dealership near me and the guy there had a couple of tanks he had kept from trade-ins. I tried to pay for them, but he insisted I take them for free!
Marty is right about the new technology, though. Be sure you are getting something compatible with your Performer, or expect to make some modifications.