I have ordered from Hosta Choice and Whitehouse Perennials, both in Ontario, both with good results. I googled "Hosta Canada" as well, and that brought up the Ontario Hosta Society, which I haven't checked but may offer more leads. As to specific hostas and maybe more suggestions for Canadian sellers, you need to get to know the Hosta Forum here on Gardenweb. I'll think about some specific ones to suggest, but the people over there can do it better and faster!

Actually, search the old threads over there; they did a bunch of surveys of favourites in each colour category in the past 4-6 weeks. I think the titles of the threads will be obvious enough.

There is a hosta forum on garden web. they could probably help you more.

If you just did a google plant search, you could see a LOT of them.
I have seen nice ones at Jumber nursery near Toronto and at Brinkmans (Brickmans?) near Stratford, but also nice ones at Hortico near Waterdown. All have sites, I think

If you look at a thread on the Hosta forum called How Do I Become a Hostaholic, you will find one of the respondents posted a list of the 50 essential hostas for beginners.