Try the mail-order companies Garden Import or Lost Horizons.

You can also try Hortico near Waterdown Ontario. Coincidentally I ordered some Trillium plants last year from them. Don't know if you can get anything this late in the year however.

Botanus has them as well.

Fraser's Thimble Farms on Salt Spring Island in B.C. has over 20 varieties of Trillium plus lots of other delectable goodies. They have an online catalog.

Humber most likely will have them as well

Wildflower nurseries like Wildflower Farm near Orillia, Ont. usually sell trillium plants, but do not ship live plants by mail. Where in Canada are you? It would be good to buy plants from someone in the same region as you as they will be adapted to your climate, etc. You could try searching for native plant societies in your province and they will be able to give you some local sources of native plants (NOT harvested from the wild). NANPS is the North American Native Plant Socciety, but based in Toronto. Their website might be a good place to start searching. If at all possible, I would recommend buying plants in person, rather than by mail-order. Expect to pay a minimum of $7-10 a plant for seed-grown trilliums. If they are cheaper than that, they are likely dug from the wild.

Here is a link that might be useful: NANPS