On the US ebay.com there are some sellers that will sell T-mobile USA prepaid sim cards internationally but shipping charges are higher (saw US$4.95 to US vs. US$8.85 to UK) plus any customs charges and duties are payable upon delivery. Also the sim will expire with 1-2 months of purchase on most cards that are shipped with a live phone number unless you add airtime (which will extend it up to a year as some others have mentioned). Suggest purchase shortly before travel.
Activating service at a T-Mobile store will cost about $50 with a sim card.
Details on T-Mobile USA sim card purchase and activation:
http://wiki.howardforums.com/index.php/Activation
Phones/Wireless Devices: Doogee S90, Isatphone Pro, Amazon Kindle 3G, SkyRoam MiFi device, Karma MiFi device, AT&T Liberate MiFi device
Sim Cards: T-Mobile (Mint), AT&T (Mifi device or Kindle), Koko
Satphone: InMarSat
Broadband US Wireless Data: AT&T postpaid, Sprint (Karma Mobility prepaid)
Broadband International Data: SkyRoam
VOIP: Skype