I know this is an old post, and I hope that by now you have found a replacement belt. For future reference try these guys located here in the U.K.
I own a belt driven OX by ooznest, it uses six mm belts. They sell them on their web site. I have found them to be very respponsive via email whenever I have had questions.
Here is a link to their belts for sale section of their web site: http://ooznest.co.uk/3D-Printer-Mechani ... ts-Pulleys