Have you thought about making one yourself? You could save a lot of $$ building it out of easy to find materials. This site offers a book that provides plans on how to make one:

http://www.willbell.com/tm/dobtel.htm

The book recommends a steel frame for the mirror mount, but you could use high quality plywood. My 14.5 inch mirror is mounted on a wooded frame.

Bill
http://my.treeway.com/observatory1