You can try bike specific sites. For example, you want a Ninja 500, check kawiforums.com and ex-500.com, or you want a streetfighter, check customfighters.com. Also, check the classifieds at www.cycleforums.com. I bought my Ninja off that site for a pretty decent price.

Finally, when using craigslist, don't be afraid to travel in order to get the bike you want. Figure out the maximum distance you will go, and then figure out what cities/states lay within your given radius. Good luck and have fun.