I haven't bought there yet but Sewclassics has a great reputation here on the site. This would be what you want most likely.
http://shop.sew-classic.com/Belt-15-...077-193077.htm
This belt is for solid hand wheel Singers. If yours is spoked you'll probably need a different size.
There was another thread on here saying the orange belts could be hit or miss on quality and to avoid the rubber band stretchy belts due to the constant tension on the motor bearings.
Hope this helps,
Rodney