What you might have seen were the top up cards to add time to the cards. I have gotten the prepaid SIMS at TMobile stores. All of the corporate owned stores seem to have them. Ebay sounds good too, although I havent dont that.

One thing to be aware of with TMobile prepaids. If you put $100 on the account, and it doesnt have to be at once, the account becomes a gold account. Anything after and the time you buy is good for a year. It is a pretty good deal if you are planning on her using it regularly.