I've never seen this tree for sale, and even looking on the web I don't see a source.

Your best bet might be to grow from seed that you collect.

check out Outback Nursery.

thanks tom and leftwood,

outback did not have the wafer listed in their catalog so I ordered one online from this vendor:

http://www.bonanza.com/listings/NATIVE-ORNAMENTAL-TREE-Hoptree-aka-Wafer-Ash-Ptelea-trifoliate-Butterfly-Must/84692443

I'm hopeful it will be a good tree! It arrives next week...

Reeseville Ridge nursery has a lot of cool natives at VERY reasonable prices and they have highest ratings from Daves Garden.