The best place to buy tubers is from local growers. Check out any nearby Dahlia Societies, etc, as some members will sell their tubers and you get a first-hand view of what the flower is supposed to look like. Also check your local Craigslist in the spring. I've had tremendous success with tubers from local growers, versus store-bought tubers (small, puny plants, few flowers).

There are a lot of trustworthy online sources as well. Good way to expand your selection to choose from. They usually open up sales any time between now & January then ship in the Spring. Here are just a handful of the many out there:

http://www.dahliasbylesandviv.com/catalog.html

http://www.dahlias.com/

http://www.hollyhilldahlias.com/

http://www.honkendahlias.com/ALL/

http://www.dahlias4u.com/2013%20catalog.htm

http://www.cgdahlias.com/

http://www.wadahlias.com/ (now taking 2015 Spring ship orders)

http://www.lobaughsdahlias.com/

http://www.lynchcreekdahlias.com/

I would add to the list Tall Grass Farm. Thanks for the list Darcy

The most complete list of dahlia vendors are on The Big List at the Colorado Dahlia Society website. Has great 'how to' articles, as well!

Here is a link that might be useful: The Big List