I have been using www.beltingonline.com for years. They can do customisation (such as different bore size, keyway groove, etc, for a fee) and stock the "HTD" range which is my preference for combat stuff. For the belts, you state the width (would suggest working around the standard widths) and figure out the right length for your setup. I'd recommend HTD5 9mm wide for drive, or 15mm wide for weapon stuff, in feathers.

A calculator like this can make the centre distance, pulley size and belt length calculations fairly painless: http://www.bbman.com/belt-length-calculator/

You may find some Ebay sellers (UK based) are a little cheaper on the belts themselves once you know the size you need.