Hi Karen,
Actually when I check with Martin a year + ago about belts he had been unable to source any, but maybe he has now.... so don't let me stop you checking...
But what I wanted to let you know is when I checked with him he suggested that I simply take IPA (isopropyl alcohol) you can buy at the Pharmacy and rub onto the removed belt ( just off the guide wheels ) for a few minutes, to get it to start gripping again. Its so easy and yet totally effective ... my belt has not slipped since... Perhaps you would like to try this in the mean time...
Hope it proves of use!
Laurence