Upvote:1
You can use AT&T GoPhones, a US-based sim card company, in Canada for free. The info on what phones work with the plan is here, and you can order the sim online, but I don't know if you can ship it to Montreal to meet you.
There's also an unrelated company called Go-Sim which sells cards that work in 190 countries, including Canada and the US. WorldSim is another, working in 90 countries including the two you want, and I'm sure you can find a few more to compare rates on by googling international sim cards. Looks like if a sim card is going to bill itself as international, it probably works in the US and Canada. TravelSim even has one marketed for travel to the US and neighboring countries. You may be able to get one of these before you leave.
Upvote:1
Wherever you buy the SIM you're almost certain that it will work in both countries, that's not a problem ( I use my phone all the time when I go to the US). The question will be how much roaming you will be charged. I think you'll save a lot if you don't need internet, but to give you an example my operator charges .75c a message and 1.5$ a minute when in the US.
You can buy a SIM in Montreal if you have an unlocked phone, and I know for example my operator has "travel packages" which make it much cheaper than actually roaming in the US.