I know a number of people that have a Cingular branded Treo 650 and had it unlocked. You can get a password into a site that helps you extract the unlock code. It costs around $25 to $30.
In that scenario, you can buy a Cingular one and use it with T-Mobile. You can probably an unlocked Cingular branded one for a good price on ebay.
Either way, good luck!!!
Lastly, have you ever heard of Craigslist?? I don't know where you live but
www.craigslist.org is a free site that has ads just like ebay. Check it out if you don't know what I'm talking about.