We have an old Tracfone that we bought for under $5.00 and we keep it active over the year by buying a $99 card, it calls Canada but you have to go through an 800 number to do so.
We also have an AT&T go phone SIM card for our smart phone, the monthly pay as you go charges start at $20 and the $50 plan gets unlimited text and talk and 5GB of data. You can fill monthly using the cars bought at WM or by using your CC on their website. It's pay as you go, no contract.
Both the TracFone stuff and AT&T stuff can be bought at Wal-Mart once you cross the border.
You need an unlocked GSM phone for AT&T. Stay away from T-Mobile, their coverage sucks.