In Kingston, the best I have found is Riley’s Garden Centre which is on Collin’s Bay. It’s not a dedicated native plant nursery, but I have gotten a few great plants there such as Eastern prickly pear and Joe Pye plant.
We are kind of in a dead zone - lots of retirements from people who operated native plant nurseries a bit further west of us. I’m going to have to check out the Merrickville, as that’s a really easy place to get to on the way to Ottawa!