There are very many places to buy dahlia tubers and one way to find vendors is to access the "Dahlia Big List" at
http://www.dahlias.net/dbiglist.htm At the top of the page are the sites of the vendors who are on line. The purpose of the list is to list all the dahlia varieties and then to list where they can be purchased. This is very late in the dahlia ordering season and many vendors have closed down sales for 2009 or are out of stock on many varieties. There are many small nurseries listed right along with the biggest. Most are very reliable.

I would like to recommend Swan Island Dahlias located in Canby, Oregon. I have ordered for several years and the quality of the bulbs is extremely excellent and they produce beautiful plants and excellent bulbs in the fall! Good luck!

I find these guys are good value and reliable. And they have a good selection.

Best of luck!

Here is a link that might be useful: Dahlias