Adelman Peony Gardens
Oregon grower offers several hundred varieties from an alphabetically arranged photo catalog.
http://www.peonyparadise.com/

Brothers Herbs & Peonies
Oregon nursery specializing in tree peonies. Photo catalog of home-grown and imported varieties.
http://www.treony.com/

Cricket Hill Garden
Specializes in Chinese tree peonies, also offering herbaceous varieties. Located in Connecticut.
http://www.treepeony.com/

Full Bloom Farm Peonies
Offers several dozen varieties from an online photo catalog. Printable order form.
http://www.fullbloomfarmpeonies.com/

Golden Port International
Importers of Japanese and Chinese cultivars of peony trees.
http://www.goldenport.com/