American Dahlia Society - these guy's set the rules if you're interested in dahlia competition, but even if not, this is a great site for lot's of dahlia information.  Be sure and check out the Classification Guide.  By exploring the guide you can find dahlias by name, class, color, form, originator, and/or size.

Big List - a great tool sponsored by the Colorado Dahlia Society for finding a supplier of the particular variety you're looking for.

Colorado Dahlia Society - lots of good info on dahlias.

Federation of Northwest Dahlia Growers

Other Suppliers

Pacific Northwest Dahlia Conference