Andy,
To my knowledge, there is nothing offered in an 18 cylinder double row radial engine.
The closest that you may find is the 500cc Seidel ST-14-500. It is a double row 14 cylinder Seidel engine and I believe that it is currently available overseas for $6000 euro, or about $9,250 USD. You might try top-collect.de for more information.
Here is the link.
http://top-collect.de/cms.asp?NoRese...kategorie=5912
The only 18 cylinder that I can think of, would be the Hodgson 18, but they are about as common as hens teeth. Any that might be in existance would have been created by the home shop machinist, so if indeed you ever would find one, then it may not have the polish and refinement that you are looking for.
www.ageless-engines.com
I hope that this helps,
--->Steve
... Also, you will likely find an interesting multicylinder engine on ebay every 2-4 weeks. Replica Engines made a 1/4 scale offy. Also a Knuclehead, Curtis and various others. Again.... they are sold out might come up on eBay a couple times per year. (Ususlly for about 5K).
Commissioning someone to do the work is not practical, as folks might spend upward of 2,500-4,000 hours building one of these engines. Most end up as family heirlooms but again, they do pop up from time to time.