They're known as fly cutters and should be available at any hardware store. They are only a few bucks, cheap enough to buy two of them so that you have one that cuts on the inside and one that cuts on the outside after you regrind the tip on one of them if you so desire.
back at the big bad guitar factory we cut thousands and thousands and thousands of soundholes and rosettes simultainously with a flycutter that used the butt ends of drillbits sharpened to cut the hole and rosette channels and ab platforms... it's an age old process