The closest thing to it that's readily available is a Type "B" (21/32 top width, 13.32 inch thick) , 42 inch long.. cogged belt (has the bumps around the inner side for better grip). This appears to be what the PA50 uses but i can't confirm it.. had no luck cross referencing the marks and numbers on the original PA50 belt.
Shop at a large auto parts store that has a rack of generic belts to browse through .. it should be around $6 or so ..
If you ask the parts guy behind the counter to find one it could be $25 or more. The one he finds will be specific to some application.
I've run the bike on a skinny type A (1/2 inch width) when playing around with things and didn't notice any difference.. although I wouldn't recommend it as a drive belt. the "B" belt grips better and can transfer more horsepower.