Try eBay. That's where I found mine in the past. You can get tubers or stem cuttings with nodes which grow just fine. I don't have enough time for them to bloom but if I can get them to 14' in AK, you should get bloom from stem cuttings. Here's one current listing.

http://cgi.ebay.com/Dahlia-imperialis-Rare-Double-White-Dahlia-Tree-/250784549829?pt=LH_DefaultDomain_0amp;hash=item3a63ec8bc5

Doesn't look like that link will work and I don't see how to edit a post. Just search "dahlia imperialis" on ebay.