http://www.missouriquiltco.com/easy-...-hultgren.html
here is the one i use and i love that i can have so many sizes... you do need to purchase a 28mm rotary cutter to get into the little grooves, but i find that this makes a very accurate circle... so if you cut several of one size, they will all match... I use this all the time for the 'circle on interfacing' iron on method for quarter circle blocks (Drunkard's Path), (by the way, white on white cirles make adorable snowmen... the kids all decorated their own naked snowmen blocks... moon over the mountain... lots of uses for this ruler...