In a former life I used a ton of different belts (and hoses), most of which I sourced through Gates Corporation - http://www.gates.com
But, if you need something like this and can't find it, I always used to hit up the Sourcing guys and they would find it either through the Thomas Register - http://www.thomasregister.com
, or through GlobalSpec - http://www.globalspec.com
The GlobalSpec guys are right down the road from me now, and I know several of them through professional associations - it's quite a neat thing they've got going with the Internet search and all. I looked at GlobalSpec quick and found 70 different flat belt manufacturers.