An annual re-post
Miller Nurseries. Our source for hardy fruit trees, grapes, and more.
Musser Forests. Our favorite source for seedlings for your farm, game preserve, conservation land, or country place, including Christmas tree plantings. Lots of native plants.
Seedland. Great source for large-volume lawn, pasture, game preserve, farm, etc. seeds. I buy meadow clovers from them.
For garden seeds, including heirloom vegetables (h/t, readers):
Renee's
Johnny's
The Cook's Garden
Gary Ibsen's Tomato Fest (These sound wonderful, but tomatoes from seeds is too much hassle for me)