You have many options because T-Mobile is GSM and you can just put your SIM card in another phone.
You can buy a prepaid phone from Wal-Mart, Target, etc. These are cheap, low-end but good phones with no contract. And you can sell the prepaid SIM and activation kit on eBay for a little bit of money back.
You can buy a T-Mobile branded phone through the likes of eBay
You can buy an unlocked GSM phone. It NEEDS to have the 1900 band, and it SHOULD have the 850 band as well (T-Mobile doesn't use it but some of their roaming partners do).
If you're on a MyFaves plan it's a little more restricted. Most phones do not have the MyFaves firmware. No unlocked and unbranded phone will. Most prepaid kits won't (but some newer ones may have phones with the MyFaves firmware on them - and they won't be marked to let you know). Be careful even when buying a phone listed on T-Mobile's site as MyFaves - only the latest T-Mobile branded firmware versions have MyFaves. Even older T-Mobile branded ones (and many current prepay kits) won't have the MyFaves firmware.
It's a simple SIM card swap to get it working.
If you bought a phone unlocked and not setup for T-Mobile, then go to tmobileusotw.wdsglobal.com to send the phone the T-Mobile data settings for multimedia messages and T-Zones (WAP)