Yes, any T-mobile (or Cingular) store will sell you the sim. For T-mobile your phone will need to be able to do GSM1900, for Cingular you'll need GSM850/1900.

Last time I purchased a Cingular sim it was free if you purchased a top-up card, T-mobile charges $25 for the sim and requires a $25 top-up card.

They will assign any phone number you want, though you can change this for free.

You can also order one off ebay.