I live in Toronto. I can't find living oysters that:
got NO worms and parasites.
I followed this video, these instructions and Jamie Oliver's to steam Crassostrea virginica bought at Loblaws (like those branded Malpeque or Raspberry Point), but meat shrinks to teaspoon size. What did I bungle?
To answer the questions.
You can use whatever fresh oysters you have on hand; smaller oysters will result in smaller yields, larger ones, mostly west coast oysters (for example Kumamoto ) will have a more mouthful results.
About sourcing oysters, most better supermarkets in eastern-ish canada will have "generic" Malpeque or Beausoleil on stock when in season.
Oysters are robust little bastards, they keep a long time, I would not worry about getting them from supermarkets.
If you cannot or do not want to get them from there, you should be able to mail order some (google for purveyors); for small quantities it might not be cost effective, but for larger quantities (100+oysters) it can be.