eBay or Craigslist, honestly. I've seen them for as little as $100 to $125. As far as a brand-new one, though, most places are all going to be about within ten bucks of one another. Summit Racing or Jeg's would be my recommendation, unless you can get a good deal through a local speed shop or similar joint.
Btw ... just an FYI, shopping threads like this go in 5.0 Talk, not Tech.