Yes T-Mobile and AT&T sim cards are readily available from eBay as Bylo mentioned. However, AT&T coverage is vastly superior to T-Mobile coverage.
On the CDMA side, Verizon Wireless is the king of coverage in the USA.

Sharon said:

I am looking to buy a cell phone and a "pay as you go plan" for emergency use. I do travel to the USA and would like to be able to call home (Canada) if necessary. I know some plans are "just for Canada" Can you help me with a plan and the best phone for my needs.
Thanks,
Sharon

Hi Sharon,

If you are going to the USA, I recommend either Tracfone, or Speak Out Wireless in the USA.

Like Speak Out in Canada, you can only buy the service through a physical 7-Eleven store. Tracfone, however, is readily purchased at many retailers in the USA such as Wal-Mart, Target, Dollar General etc.
Just like Speak Out Canada, the USA version's top-ups all last a year, and they are a bit cheaper. The lowest top-up amount you can get in the USA is $10.00
Check out this link:

http://en.wikipedia.org/wiki/7.....t_Wireless

Tracfone is good as well, but I think Speak Out would be more as an option for you, and is cheaper than Tracfone.

As I travel frequently to the USA, I signed up with Tracfone a few years ago. It is a good service, but I changed to Speak Out Wireless USA, because like you, I only use my phone while I'm in the States for emergencies.

As far as recommending a particular phone, any unlocked phone that works in Canada will work with Speak Out Wireless in the USA.
When I got my Speak Out USA package, a Nokia 1600 was included. It is a great basic phone.