R. calendulaceum is the flame azalea. It is one of the most brightly colored of native North American shrubs, bearing large clusters of 2-inch flowers in early summer. Plants usually grow 4 to 9 feet tall in Zones 5-8, hardy to -10° F, and occasionally reach 15 feet. It bears clusters of 2-inch clove-scented bright scarlet, orange or yellow flowers in late spring or early summer flowers that are long lasting, even in full sun. The leaves are 3 inches long and drop in the fall.

A couple sources are:

RareFind Nursery in Jackson, NJ

Girard Nursery in Geneva, OH

Eastern Plant Specialties in Rahway, NJ

Weston Nurseries in Hopkinton, MA

Hope this helps

Here is a link that might be useful: Native Deciduous Azaleas

debc...
Here's a nursery that specializes in the Eastern US species of Native Azaleas and where I purchase mine.

They also have Wholesale prices for large orders.

Rhodyman...
Just returned from several days at the Phila Flower Show. Spoke with a couple of interesting gentlemen from the Phila Chapter of the ARS, but never observed anyone from your Chapter.
Overall great show this year. Our 12th year working and judging there. I'm getting too old for the airline travel grind (weather delays, cancellations, rebooking, substitute carriers, etc. and then there's the TSA!), so it will probably end soon.
Hope you had the opportunity to attend, if not, there's one day remaining!
Rb

Here is a link that might be useful: Lazy K Nursery

Couple of other suppliers are:
Mail-Order Natives amy@sumnet.tv
shop.shadygardensnursery.com

Both of these I would highly reccommend.