I've bought a few belts from CPC Farnell.
If you go on their website and enter 'Drive Belts' you'll see they've got quite a range.
They give all the dimensions - but knowing what yours should be is the key of course.
Flat belt widths and depths are straight forward enough - diameter, taking account of stretching, not so clear cut - but there are guides on calculating....even if yours is broken.
Of course, you'll be much better off doing what Al says above and getting a genuine replacement.