I live in Mississauga, so I can help out a bit. There's cigar shops in a grocery store called Loblaws. If you can find a Loblaws, you can find the cigar shop, since it's right at the entrance. There's also two cigar shops in the local mall, Square One. Square One might be easier to look at, though.

And it is 19 to buy smokes. But to be perfectly honest with you, I've never been IDed when buying smokes or alcohol. The only place I get IDed at are bars. So don't be afraid of the warning signs that say something along the lines of "If you're 25 and under, you'll be carded".