Seed Savers Exchange is literally, in my opinion, the best resource in the world for heirloom seeds.
First, they offer a color catalog full of heirloom seeds... and what a catalog! This catalog contains dozens of varieties of tomatoes, dozens of varieties of peppers, over a dozen types of melons, over a dozen lettuces, and a wide variety of almost any traditional vegetable you can name. However, all these plants are heirloom varieties that are open-pollenated (I'll discuss this more in a future post), which means you can save the seeds to plant again next season. If you only were able to order seeds from this catalog, this would be a really good resource. Take a look at their catalog online here. You can order seeds from them online here.
Seed Savers Exchange Heritage Farm
However, what makes Seed Savers Exchange such an amazing organization are their Yearbook publications (summer, harvest, and spring). These are listings of home growers (and what they grow) across the nation, a network of backyard seed savers, who have collected and conserved heirloom seeds and plants. This listing gives you access to plant varieties that have been selected, sometimes through decades of selection, to grow best in that growers local climate. This now give you the ability to find growers in your area and obtain seeds or plants from them that are ideally suited to your environment. There are also varieties of plants that local growers develop that you will never find in a catalog. This is an invaluable resource.
Seed Savers Exchange Lillian Goldman Visitors Center
When I lived in Minnesota, I had many opportunities to drive down to Decorah, Iowa and visit their Heritage Farm and Historic Orchard full of over 700 varieties of apples (I was able to taste well over a dozen that you can never find in stores!). If you live anywhere near this area, I highly recommend a visit. You can easily make a day of it. Just bring a picnic lunch and spend the day walking over their land. Fantastic!
As you can tell, I am a huge fan of this organization, and I cannot recommend them highly enough.