I was actually on ebay before coming on and was searching for ideas on a similar project. There is actually a pair of 27" wheels with flip flop hubs available, if you want to keep your brake setup. As far as i know, besides having a set built, this is the way to go. There is also a rear single, if you want to keep the front and just replace the rear.

http://cgi.ebay.com/NEW-27-ROAD-BIKE...QQcmdZViewItem