In Montreal there are several fish stores, but even though you feel that something is missing from your collection and the local pet store cannot offer.
For the CO2 the best was build one using a CO2 Paintball tank following a thread from this Forum.
But luckily I now a really good fresh water fish store in Montreal called AquariusWeb and they are magnificent when looking for health fishes. Good collection and reasonable prices.
Also, I found that buying from Big Al's Canada and other big on-line stores when not paying shipment is a best practice.
CANADA is AWESOME - ps. I am Brazilian Born