This will tell you want range of rim widths you can use:
http://www.schwalbetires.com/tech_in...dimensions#rim

Check around on eBay,you'll occasionally find takeoff wheelsets. If you can stand to have mismatched wheels,you could always just get the rear rebuilt with a new rim. If there are any co-ops/bike kitchens in your area,they could teach you how to build a wheel yourself.