There is a wonderful site called helpmefind.com. You can look up roses (and clematis and peonies), read comments from people all over the world, see real garden pictures and see sources for any plant listed by clicking the "buy from" tab.
Does AllThingPlants have reviews of suppliers? The site Dave's Garden has individual ratings of different plant suppliers.
I think this is the best rose forum on the internet and the rose database is wonderful. I don't mean to detract from this site by posting about other sites. Just trying to be helpful.
The Canadian nurseries use hardier rootstock as well. I haven't bought roses from them yet. I am focusing on own root roses except when tempted by grafted plants on sale in local shops.
If you research varieties, you may find zone 4 hardy own root plants for sale in your local stores. I saw several at Home Depot today. You could look them up in the AllThingsPlants rose database with a cell phone other high tech device while shopping.
