Canadiens.NHL.com
The official source for Canadiens tickets is the NHL.com. The site is slow and buggy, but if you can get through to look at the ticket selection, you can find them for face value (the price printed on the ticket). They sell out fast, especially for playoffs games, which means this is often only the first of many shopping stops when you need tickets.
TicketLiquidator
A faster, easier web site that never seems to sell out of tickets is TicketLiquidator.com. They are what's called a "ticket broker". I don't exactly know what that means but from what I can tell they basically buy tickets from people who aren't going to use them and then resell them for a profit. Sometimes the prices are outrageous, but sometimes they're absolute bargains.
E-Bay
If for some reason you can't find the tickets you want on TicketLiquidator, you can try Ebay. But I don't know if I fully trust this type of auction buying.