There are a number of options for iPhone telephoto lenses, and not all of them are of a great quality. Many have design issues, or just seem cheap, gimmicky, or hard to use. My recommendation for the very best iPhone lens system out there is the OWLE. Sorry, I can't post a link yet as I lack the required number of posts, but a simple Google search for "OWLE" will pull it up. They have a version for the 3GS, and also one for the iPhone 4.
As far as where to buy, I tend to stick to either the manufacturer website, or major sites like Amazon, Best Buy, Buy.com, Newegg, TigerDirect, etc. I've had too many bad experiences when I risk purchasing from a small site, or one I've never heard of before.