These plants are, of course, the official flowering plant of Ontario.
Many are out in full bloom now on the Niagara penninsula, including some purple ones too. Unfortunately, the purple ones were not as photogenic; all the ones we found when hiking were facing downwards as if they were top-heavy.
These plants grew in the back of a farm I lived on for sixteen years, it was always a nice treat to go back and see them in the spring, knowing I was probably the only person to see the flowers, because no one else went back there.
If you're looking to find some, look in any mixed forested area in Southern or Mid-Western Ontario, anywhere in the US that is located Arkansas and up. Not too swampy, and not all evergreens, and you should be able to find some!
While I don't have as much of an affinity to flowers as many other people do, trilliums (trillia?) I've always appreciated. Perhaps it is because not as many people grow them as garden flowers, due to the difficulty involved, so I'm not used to seeing them everywhere. There's something to be said about this plant more often being seen in its natural habitat, rather than artifiicial ones.