i have used ebay and Swappa (Android) Buy and sell Android devices
swappa is a great site think there are 5 thunderbolts for sale on there right now, just have to have a paypal account to buy!
As long as you do not mind lightly used you can save some $, i have yet to actually buy a android phone on contract, i bought my eris on ebay and both my Inc and Fascinate on swappa. Havent had a bad experience yet, just make sure that there is photos of the phone itself and not "stock" photos and that the seller has a 100% satisfaction rating on ebay. Good luck!