Where can I upgrade my t mobile phone for free? I know walmart carries their phones but who else does?

Best buy. Frys, target

I assume when you say "free," you mean you want another T-Mobile phone without paying for the upgrade charges the phone companies do, and you don't want to extend your contract at this time. Am I right?
There are a lot of places you can purchase your phone, without having to pay the overly expensive prices the phone companies charge. In this day and age, I'm surprised they still charge those prices! And it's really easy to activate an unlocked phone; you just call your cell phone provider or go online, and it's activated. Many companies don't even charge you to do it!
I've had some luck on eBay finding phones, but I've also had some real lemons. My last phone, for example, took a whole month to come, and the battery cover keeps falling off! Good thing I got it for $20 or I'd be pretty annoyed by the whole experience!
If you're looking for the top phone out there, http://www.buy.com has the T Mobile 3G iPhone. It's not exactly free though; without the cell phone plan it will cost you close to $700.
Wanting something that costs less? Two places, ebay and J&R Music have the Sony Ericsson pictured for about $30. It's not an iPhone, but you can't beat the price!
Coming in a little higher is the Samsung T809 GSM Slider. It will give you bluetooth, a MP3 player, speakers and a camera all for around $100.