I ordered two peonies from The Peony Farm in 2008. I am pleased with the root stock they sent me. The plants bloomed the following spring, and are looking even better this year.
Their web site is low-tech; to place an order, you print an order form and mail it to them along with payment by check. Before mailing them my order, I e-mailed them to confirm that the peonies I wanted to buy were still available. They responded to my inquiry promptly.
They offer tours of their farm during the blooming season (see their web site for dates).