You can probably find them on Newegg. If you're building a new computer from scratch, then the i5-2500K processor is the best. If you're into heavy video processing and other CPU-demanding tasks, then look into the i7-2600K processor.
Edit: it appears that Newegg doesn't have the Core 2 Extreme processors any more - I just checked. Have you tried searching Google or something?
The Intel® Core™ 2 Duo and Intel Core 2 Quad processors have been discontinued so finding them should be problematic at this time. You can check with pricewatch.com or if you must have one of these processors there are some resellers out there like http://www.cuetech.com/ who can find hard to find products. When it comes to ebay that is up to you on buying from someone there.