In Canada, before the merger, Canadian Airlines usually served Labatt's Blue Pilsner (I don't like this beer), while I've seen Molson Canadian and Molson Dry (now, THESE I like
) only on Air Canada flights. I know because on CP flights, Molson-brewed beer is usually NEVER available! So, I'm pretty sure that before the merger, Air Canada had a contract with Molson, while CP had its contract with Labatt's breweries.
WestJet will serve beer even though it is a discount airline, but only on flights one hour or greater in length. It serves both Canadian and Labatt's Blue. But interestingly, I think I've seen Big Rock beers served on WJ flights. if that's true, usually it's Grasshopper or Traditional Ale. Like WestJet, Big Rock is also headquartered in Calgary.
It usually costs about 4-5 dollars to buy any alcoholic drinks, including beer on all AC flights in economy class. But if there's a delay of more than an hour, then they'll serve the first beer for free. I don't know if they do the same on US carriers.