When I first grew dahlias (~2005) I grew them all by seed. Here in Michigan, they must be started indoors and planted out after the last frost. They will bloom the first year.
Bedding dahlias are probably the easiest to grow and are available from many seed companies.
I found a wider selection of different dahlia type at Thomson & Morgan Seeds. Also, Swan Island Dahlias offers seed, but I have never tried them. I have purchased many tubers from Swan Island, just no seed.
If I remember correctly, I saved entire dahlia clumps to over winter them. Many of the clumps got fairly large and a single clump would barely fit in a standard paper grocery bag.
