+1 on Peter's recommendation of the MFS. Super accurate and flexible.
If this is for a one-off project, they are easy to make. Use some plywood scrap. Use the base of your router to mark where screw holes should be drilled. Get some longer screws and screw the router to the end of the ply. Plunge down to reveal the bit location. Then measure the radius from the inner or outer edge of the bit, drill a hole for the pivot (nail, screw, rod, whatever) and you are done. Need another radius? Drill another pivot hole.