Prairie Moon Nursery
Prairie Moon Nursery has a massive collection of over 700 North American seeds and plants. It has an excellent plant finder and displays the Native Range of plants in addition to invasiveness.
Native American Seed
Native American Seed specializes in North American plains plants. They have a small but mighty selection of native wildflowers, grasses, unique conservancy species and mixes.
Grow Milkweed Plants
Grow Milkweed Plants is an excellent resource of all things Milkweed. You can search for native Milkweed species throughout Canada, the US and Central America by State Province or Territory.