Fairweather Garden in NJ, Whitney Gardens in WA, Rarefind Nursery in NJ. Many others as well. Just Google "deciduous azaleas for sale,"

There is a vast number of hybrids and species, including nearly all the native North American ones, that should do just fine in your area.

Deciduous azaleas are well known for their bright colors and cold hardiness. However what is less well known is that some get mildew on the leaves around mid season. Here are some that do well in SE Pennsylvania and should do well for you. Listed is (series), height in 10 years, cold hardiness, etc.

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Candy Lights (Northern Lights) 5', -35F. Fragrant, Mildew Resistant.
Gibraltar (Exbury) 5âÂÂ, -25F, Mildew Resistant
Klondyke (Exbury) 5', -20F,
Millennium (Weston) 4', -20F. Mildew Resistant, Fragrant
Narcissiflora (Ghent) 6', -15F, Mildew Resistant, Fragrant
Tri-Lights (Northern Lights) 5âÂÂ, -25F, Mildew Resistant
Weston's Innocence (Weston) 3', -25F. Mildew Resistant, Fragrant
WestonâÂÂs Lemon Drop (Weston) 4âÂÂ, -35F, Mildew Resistant, Fragrant

Note, Klondyke is not listed as mildew resistant, but mine seldom gets mildew and we have morning dew just about every day as you probably do also.

Here is a link that might be useful: Deciduous Azaleas

You're not looking hard enough. http://findnativeplants.com/mid-atlantic/maryland-native-plants/

They're available and you do not have to go mail order.