There are many dahlia ressources on the net: commercial and personnal pages, dahlia societies, illustrated guides, forums, growing tips.... Here are a few links:
La Société québécoise du dahlia. One of only french-speaking dahlia society (and web page) in North America. It contains articles, an illustrated guide, and a list of the society's activities.
The American Dahlia Society. The number one reference on dahlias in North America! Many links, a growers-forum, dahlia-net, and an illustrated guide containing more than 1000 photos of today's best dahlias.
Dahlias in B.C.. A site built and maintained by one of Canada's foremost hybridizers, Mr. Wayne Holland from British Columbia. Lots of interesting articles and pictures of his latest hybrids.
Cal's Dahlia Page. A personnal web page that holds a lot of dahlia photographs. Also a commented list of dahlia related links.
The On-Line Guide to Growing Dahlias. Everything you have to know about the basic rules of dahlia growing. Many tips, from the selection of tubers to the preparation of the blooms for the shows. Also has many links.
The Illustrated Guide to Growing and Showing Dahlias. A site developped by two major dahlia societies containing many well illustrated and documented articles.
Frenky's Dahlia World. A dahlia perspective from the Netherlands. Check it out, just for the beautiful home page!
My wife and I a few summers ago at the Jardin des Quatre-vents in the Province of Québec, Canada.