Note: Seed Catalogues are as seductive as a Pottery Barn Catalogue. Underneath the fluff is some crafty marketing that takes advantage of a gardener's vulnerability: overreach. "New" is better than old, stripes are better than plain, ornamental is preferable to flavor.
Ordering seed is even more frustrating if your favorite varieties are spread over a dozen different seed companies. This gardener's answer was a table I print out every year to determine what seeds I need to order from which company. It is a Microsoft Excel table that can be opened up in Microsoft Word and printed. Right click on the file and save it to your hard drive in the appropriate folder.
Cooks Garden, Johnny's, and FedcoSeeds are the best catalogue sources for organic and heirloom seeds. See Garden Links for other online sources. Johnny's and Stokes catalogues contain the best sources of growing information. Stokes sells many flower packets separated by color. Harris sells a lot of old standards. Fedco Seeds (Maine) is another source for organically grown, heirloom, and open pollinated seeds. Fedco is the only seed company that codes its seed sources according to size of company and acknowledged involvement in genetic engineering. Shepherd's used to be a favorite, but they have apparently dumped seeds and morphed into the pricey White Flower Farm.
See Planter's Favorites for more on individual cultivars.