I have to agree with Prairie Moon. Too bad there aren't any similar nurseries for the Northeast. (There is the New England Wildflower Society, but they sell for only a short period and at rather high prices.)

I have found good selection at the Canadian store gardensnorth.com, but they are expensive!

Ion Exchange

Easy Living Wildflowers in Mo. Real easy to deal with. I send email to John with what I want and now he ships them before he gets my check. I bought a packet of Little Bluestem seeds and got about 10,000 instead of 50.
I order from them every year. This year I even broke down and bought plants. They were packed incredibly well.

Here is a link that might be useful: Easy Living Wildflowers

Ernst Conservation Seeds.

Here is a link that might be useful: Ernst

maybe funny but i got some great poppy mix on ebay

For God's Country (or Hell's Half Acre, depending on your perspective), Spring Hill Nursery.

Here is a link that might be useful: Texas Seed Source

i've since learned that spring hill is not as reputable as i believed

so scratch that recommendation