I have Nokia Lumia 521 (originally ATT phone) for international travel - bought it while I was with Sprint. Was able to unlock it through friends account. It sure does the job and Win 8 is not that bad, actually.
Tmobile has Lumia 520 - prepaid phone under $100. I would check with TMO how easy it would be to unlock it - if they do it - it's a great small phone for travel.