Yep spotties are pretty expensive...they used to be a common turtle in the wild (not incredibly common but I used to be sure to find a few each year) but not any more. I think they're protected through most of their natural range so the ones you buy should be CB for sure.
You could also try a eastern painted turtle (Chrysemys picta picta) as their wild range extends into Canada - they will be able to handle the cold. Be careful though, most of the paints you see for sale are Western or Southern and will not be as cold-tolerant. I mentioned the paints because IME the spotties like a bit more land area than most basking turtles do. They're not as terrestrial as some other Clemmys species (like C. insculpta, the American wood turtle) but moreso than paints & sliders.
i just checked my field guide and both the easter and western painted turtle and also the midland painted turtle should do alright. size is 4"-9 7/8"
i think stinkpots or spotted turtle might do alirght too. i know nothign about them but the field guide says that the spotted turtle is 3 1/2" - 5" and the stinkpots are 3" - 5 3/8"
