This should be easy - Canada has the same numbering system as the US (and the same plugs!), had a Bell System, and is so close. But nooooo. Don't be fooled. AT&T does not directly work here, and you get raped if you roam, and still reamed if you buy an "International" package. So when you land in Airplane Mode, don't turn it off.
I found a way, a clever hack. First I wandered Old Montreal for a phone store or kiosk. Montreal has the largest underground city in the world, the RESO - after all, it is so far north, in winter it is so snowed in you expect to bump into Santa's elves - and I meandered northward mostly underground into a mall by the Hyatt. Hmm, despite four phone kiosks and one store (Le Source), no killer SIM option - although the Bell Canada "flex plan" gave me an idea.
I then headed west (Ouest) above ground on the delightful Rue Saint Catherine to the mother of all phone kiosk places - Eaton Centre - a modern mall inside an older building. They are all there (Fido, Telus, Bell, Virgin, Rogers, Videotron, etc).
The prepaid plans for Fido and Telus are C$10 for the SIM and then $10 or $25 for data depending upon how much you want. $20 total ain't bad, but if you go over you don't roll into the $25 plan, and need to top off.
There is a better way. A hack.
I bought the Fido $10/mo no-contract data SIM with a credit card. This is a flex plan which rolls into the next tier if you go over. I got 100MB for just $10, and if I go over it is $25 for a boatload more data, automatically. No spurious $10 for just the SIM either. Just $10 all in.
All I have to remember is to cancel after I leave, to avoid a $10/month continuous charge. That may take a phone call. Easier than topping off.
Telus had a similar plan but the entry tier is $15 for 150MB. Bell had a similar plan starting at $5/mo for tablets that also work on GSM phones but requires Canadian residency.
The Fido plan requires a passport. Bring it!