I have can recommend these mail-order native plant sources for native seeds and plants.

Prairie Moon Nursery
Native plant seeds currently super-hard to find anywhere else like our native Tall Thistle (Cirsium altissimum)

Wetland Plants, Inc.
They specialize in wetland and aquatic plants and normally do wholesale only. However, if you know exactly what you want, they will sell plugs to the home gardener. I put in a sedge garden (sedges host many butterfly caterpillars) with plants from this wonderful nursery.

Diana (males on right and center) and Great-spangled Fritillaries on Wild Bergamont (Monarda fistulosa) in Haywood County, NC.
Adult butterflies LOVE mass plantings of native wildflowers.