There are a few Canadian seed houses that I've dealt with that mail started plants and Perennials.
Dominion seeds, Vesey's seeds and T+T seeds. They all have on line sites.

As for greenhouses in your area I don't know but they should be listed in the phone book under garden centers or Nurseries. I know my son lives at the other end of Ottawa and there are several that he visits.

Another source would be to call would be a horticultural or garden club and ask there.

Great ideas, thank you!

My old neighbourhood!
The Home Hardware in the Glebe sells some plants on the sidewalk during the spring and summer, and there are a couple growers at the Farmers Market at Lansdowne (check the days online, I think it's Thursday and Sunday). The Parkdale and Byward Markets also have plant sellers - you can probably take a bus there.
Good luck!