If you have an unlocked Windows Phone device that you want to bring over to T-Mobile’s network in the U.S., you can get a SIM card for free with a coupon. That’s with free shipping, too. Just wait for the SIM card in the mail, choose a plan, and put the SIM card in your phone.
T-Mobile’s SIM cards are usually sold for $10. Head over to their online store and enter coupon code: NOVSIM. There appears to be a limit of two cards per customer.