cowboom.com. its affiliated with best buy, and they have a decent selection of recent phones. Prices will vary by phone, but if you check there regularly you can catch some pretty good deals.
I just got my Bionic about a month ago for $170 (used/refurbished). It honestly looks no different than a brand new phone, there isn't a semblance of a scratch on it lol and I still have my upgrade from vzw to use if something dope comes out in the next year or so.
PS if you're worried about ESN numbers, they let you return any phone within 30 days, but I don't know if there's a way to check the ESN of the phone you're getting before they ship it (my bionic from them had a clean ESN).