Mobile Internet options for Canadians in the US and Mexico?

You can bring your Shaw TV service you to the US and it will work all the way into Mexico. You need the same Shaw antenna like the one on your home which you will put on a tripod and point at each new location. It sounds challenging but it's not after a bit of practice. Setup should take less than 30 minutes once you understand the process.
You should be then be able to use something like Straight Talk from Wal Mart. Not cheap but good in the US. They likely have a Mexico plan available as well.