Black V-belts are best
 

Many vintage sewing machines got a belt between the motor and the flywheel of the machine. You will find these belts and some pulleys as replacement parts from many stores. The belts will typically be black V-belts, colored lug belts or round O-rings. This video show the result of a comparative tests with these kinds of belts. The black V-belts got the lowest friction loss in the comparison.

https://youtu.be/pGHbPfaSXl8

Did you know of similar test results elsewhere?

Does anyone have a good source for V-belts of decent quality/price? I have ended up refurbishing and selling a lot of my machines and if I need a new belt it's a round O-ring because of the cost.

I think the sewing machine stores will normally have them, but I often find limited information on the websites on what the exsact dimensions are. Try look here:
http://www.ebaystores.co.uk/simplybe...on-Wizard.html