Answer: According to the American Horticultural Society's book, "Plant Propagation" it's best to sow seeds and keep at a minimum temperature of 61 degrees F at all times to get rapid germination. You can use a heated seed mat to ensure even temperatures. When seedlings are larger enough, transplant into individual pots. Plant outdoors when temperatures are 54 degrees and above.
As for seed sources, I did a plant source database search and nothing showed up for seeds, just plants. But here are some dahlia specialty nurseries that you can check with directly. You can also try saving seed from your own dahlias or from friends.
Dahlia Dandies, 1717 South Woodland Drive, Kalispell, MT USA 59901 http://www.valler.com/dahlia
Alpen Gardens P.O. Box 63 Lynden, WA USA 98264
Swan Island Dahlias Box 700 Canby, OR USA 97013
Bridge View Dahlia Gardens 1876 Maple Street North Bend, OR USA 97459
Hilltop Dahlias 1010 Antelope Street Basin, WY USA 82410
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