this answer is a bit late, but I thought it would be useful to others who may be asking a similar question.
I recently returned from a trip to the USA.
It is easy to buy a Sim Card over the counter, and you can get one from any of the carriers
The largest being:
- Verizon (CDMA) (Largest)
- AT&T (GSM) (Largest 4G at time of post)
- Sprint (CDMA) (Update No Prepaid)
- T-Mobile (GSM)
Most will have prepay options, but it may be cheaper to get the Pay Monthly options if you are there for close to a month (or more).
Most of them have a $1 -$3 per day type prepay plans Or $40-$50 per month unlimited prepay plans.
Which one you chose depends on which cities you are visiting (different coverages) and what kind of phone usage you think you will need (calling, txting, data) are you using a smartphone? do you need 3G or 4G?
If you have a smartphone, a good way to save on data charges is to connect to free wifi hotspots at McDonalds and Starbucks which are plentiful in the major centers (both USA and Canada).
The USA sim card will work in Canada but you will be subject to roaming charges
My personal experience is that T-Mobile doesn't provide very good 3G coverage throughout the country, often falling back to 2G. So I would recommend going with AT&T if you have a GSM phone, and Verizon if you have a CDMA phone.
For a full comparison of the carriers: