Seed Savers Exchange:We have ordered a lot of seeds from this site and are big fans. According to their site, ther mission is to "conserve and promote America's culturally diverse but endangered food crop heritage for future generations by collecting, growing, and sharing heirloom seeds and plants."
Baker Creek Heirloom Seeds: We have been very happy with our orders from Baker Creek. They say that they "carry one of the largest selections of seeds from the 19th century, including many Asian and European varieties. The company has become a tool to promote and preserve our agricultural and culinary heritage." I like there variety and their back story as a family business.
Johnny's Selected Seeds: Although we haven't ordered from them we have heard a lot of good things. They have an organic and an heirloom section. If you want to harvest seeds make sure you look for heirloom varieties.
High Mowing Organic Seeds: This company sounds intriguing as they say that all of their heirlooms are organic.
Territorial Seed Company: Established in 1979, they seem to have a lot of variety.
Another suggestion is to find a local gardening store to see if they carry heirloom seeds. There is a company called IFA here in Utah and they already have displays of heirlooms that they say have been proven to grow well in our environment.I like the idea of going to a local company if you can.
What have I missed? What other places would you recommend?