My website lists all 26 native rhododendron and azaleas in the USA. It is at:

Common Native Rhododendrons and Azaleas of North America.

Hank Schannen's Rare Find Nursery in New Jersey carries the following native azaleas and rhododendrons:

R. (AZALEA) alabamense 'Frosty' AKA 'Terry Greer'
R. (AZALEA) arborescens
R. (AZALEA) atlanticum
R. (AZALEA) austrinum
R. (AZALEA) austrinum "Choctaw-Hatchee Seedlings"
R. (AZALEA) austrinum 'Dave Lewis' Yellow'
R. (AZALEA) austrinum 'Wise Decision'
R. (AZALEA) calendulaceum
R. (AZALEA) canescens
R. (AZALEA) canescens 'Camilla's Blush'
R. (AZALEA) canescens 'Varnadoes Phlox Pink'
R. (AZALEA) cumberlandense (bakeri)
R. (AZALEA) periclymenoides
R. (AZALEA) periclymenoides 'Lavender Girl'
R. (AZALEA) prinophyllum (previously roseum)
R. (AZALEA) prunifolium
R. (AZALEA) vaseyi
R. (AZALEA) vaseyi 'White Find'
R. (AZALEA) viscosum Swamp Azalae
R. (AZALEA) viscosum 'Betty Cummins'
R. (AZALEA) viscosum 'Dave Lewis Pink Volunteer'
R. (AZALEA) viscosum 'Delaware Blue'
R. (AZALEA) viscosum 'Pink Mist'
RHODODENDRON catawbiense (Mountain Rosebay Rhododendron)
RHODODENDRON maximum (Rosebay Rhododendron)
RHODODENDRON maximum 'Curly Leaf Seedlings'
RHODODENDRON minus pink form
RHODODENDRON minus 'Chatahoochi Dawn'
RHODODENDRON minus carolinianum group

They all are native plants but they are selected as being the best forms of the plants. Some are named, others aren't.

Another great place to buy native Azaleas and Rhododendrons is Jim Gear's Octoraro Farm Nursery. Jim is a native plant specialist and has the following:

R. alabamense - (Alabama Azalea)
R. alabamense 'Frosty'
R. arborescens (Sweet Azalea)
R. arborescens var. georgiana
R. atlanticum (Coast Azalea)
R. austrinum (Florida Azalea)
R. austrinum 'Calamity Junction'
R. 'Don's Variegated Austrinum'
R. austrinum 'Millie Mac'
R. austrinum 'Pretty One'
R. calendulaceum (Flame Azalea)
R. canescens (Piedmont Azalea)
R. canescens 'Camilla's Blush'
R. 'Varnadoes Phlox Pink Canescens'
R. cumberlandense (formerly bakeri, Cumberland Azalea)
R. flammeum (formerly speciosum, Oconee Azalea)
R. flammeum 'Double Pleasure'
R. periclymenoides (formerly nudiflorum, Pinxterbloom)
R. periclymenoides 'Lavender Girl'
R. prinophyllum (formerly roseum, Roseshell Azalea)
R. prunifolium (Plumleaf Azalea)
R. prunifolium 'Little Red Riding Hood'
R. vaseyi (Pinkshell Azalea)
R. viscosum (Swamp Azalea)
R. viscosum 'Summer Eyelet'
R. viscosum var. serrulatum (Hammocksweet Azalea)
R. maximum (Rosebay Rhododendron)
R. minus var. minus (Carolina Rhododendron)

A major rhododendron nursery near you is Weston Nurseries in Hopkinton, MA near I 495 and the Mass Pike. They carry a few native plants:

Rhododendron canadense 'Rhodora Azalea'
Rhododendron catawbiense AlbumÂ
Rhododendron catawbiense ÂAlbum ElegansÂ
Rhododendron catawbiense ÂBoursaultÂ
Rhododendron maximum (Rosebay Rhododendron)

There are some plants native to the West Coast that don't seem to make it on the East Coast, so be careful of ordering from a West Coast nursery. Good Luck.

To be more specific, plants which are native to parts of Massachusetts include:

Rhododendron canadense 'rhodora azalea'
Rhododendron catawbiense 'mountain rosebay rhododendron'
Rhododendron maximum 'great laurel' or 'rosebay rhododendron'
Rhododendron periclymenoides 'pink azalea'
Rhododendron prinophyllum 'early azalea'
Rhododendron vaseyi 'pinkshell azalea'
Rhododendron viscosum 'swamp azalea'

The USDA has a good site for finding which plants grow native in any area. It is at: http://plants.usda.gov/index.html

i like these growers (but they are located in the south):

huge selection:
http://www.ferngullynursery.com/

http://www.lazyknursery.com/azaleas.html

also,
Mail-Order Natives in Lee Florida

Woodlanders

&

http://www.we-du.com