Ernst Conservation Seeds has great seed mixes, plus full descriptions of how to prepare the area you plan to convert into grassland/meadow or other uses. They provide seed mixes for riparian areas, meadows, pastures, or specific needs like "to attract pollinators and birds," as well as planting details.
Contact: 814-336-2404, email@example.com
Prairie Nursery provides custom seed mixes and is committed to native plant restoration. They also provide consulting services and a variety of resource guides.
Contact: 800-476-9453, http://www.prairienursery.com/customer-care/contact-us/#.VOtXO_m-1cY
Prairie Restorations is committed to native plant community restoration. They provide native seed and plant materials, planning and educational resources, and installation services.
Contact: 763-389-4342, firstname.lastname@example.org
Roundstone Seed provides ecotype seeds that are adapted to your native region. They are experts in native seed and native seed establishment.
Contact: 270-531-3034, email@example.com
Sharp Brothers Seed Company provides seed for home, farm, and commercial landscape use. Their stock includes warm and cold season grasses, various legumes, forbs, and wildflowers.
Contact: 800-462-8483, firstname.lastname@example.org
Stock Seed Farm provides prairie grass, turf-grass, and wildflower seeds. They are committed to butterfly and pollinator conservation.