I am upgrading from my 8130 to an 8330 (vzw). I managed to snag one off of eBay for a decent price. As long as you are getting a guarantee that it is not a dead phone or a bad ESN you are usually ok buying from there.
Another advantage is more storage (memory) on the device. As for the cheapest way to get a Curve for TMobile... make sure whatever you do, you get an 8320, which is the TMobile supported Curve.
As for getting an unlocked device, you can just buy any TMobile Curve from an online store, swap out your SIM and put it in the new device and you are ready to go!! No need for an unlocked phone... especially since you want the 8320, the TMobile Curve
EDIT: I forgot to add that the cheapest way would probably be calling TMo and upgrading your phone... if it had been 22 months since your last upgrade you get the "upgrade price" which is $150 for the Curve. You could also extend your contract and I believe that make you eligible for an upgrade as well, but I am not sure...
