BrainH suggests a pair of dividers with one point sharpened to a blade. If you search for 'circle cutters ' on the internet there are many cutters using the same principal, but with a proper blade one end and used for cutting card, leather, rubber etc, priced from £3 upwards. Usually made of plastic, they work OK to cut discs out of sandpaper, just need to press quite hard on the blade end if cloth backed. However I usually find scissors good enough.