My company recently switched from Nextel to T-mobile (we're sporting 7100's now). As a result, we are selling off our old assets in the form of about 15 EACH of Nextel Blackberry 7510s and 6510s.
We've put them up on ebay and you can see them here: One of our eBay auctions
We've set the reserve at $79 for the 7510 bundle that includes the phone, wall charger, one charger/data cable only, cradle and holster, plus Nextel SIM card.
Contact me if you're interested in buying in bulk for your organization, I really want to move these and clear some space around here!