Not unlike Verizon Wireless and Sprint, T-Mobile has rolled out a large selection of Android based smartphones. The days of Verizon's monopoly on the new technology with the Motorola Droid are long gone.
T-Mobile has made itself stand out by having the best deals of all the carriers. Presently, the LG Optimus leads the way, along with the Motorola Cliq, Charm and DEFY. That does not even include the Android phones which are made in house by T-Mobile that are great values.
If you're old school and do not like buying things online, you can always go to your nearest licensed T-Mobile dealer. Sometimes it's a good idea to look at independent retailers because their deals are sometimes better. However, doing it in person just takes more time.
If you spend your time, you can find all these phones for free online. Start with Google and search for terms like "T-Mobile Android phones" or "Android phones for T-Mobile" That will give you a full list of online sources to order your new smartphone. It should also bring up a lot of review sites, so that you can make more informed choices, Even better, a lot of the review sites (like CNET or Mobiledia) give you a list of retailers and the best prices online at the end of their reviews.
Finally, you can go to places like eBay or Amazon.com if you do not want to sign onto a new contract with your phone. It will not be free but you will not be tied to one carrier, either.