Harbor Freight:
http://www.harborfreight.com/cpi/cta...Itemnumber=895
http://www.harborfreight.com/cpi/cta...emnumber=37173

thetoolwarehouse.net
http://www.thetoolwarehouse.net/
put micrometer into the search bar on the top right

northerntool.com
http://www.northerntool.com/webapp/w...11&R=200306411

several options... with rotor thickness its not that necessary to be that accurate... ballpark will let you know when to replace...

i got one from thetoolwarehouse.net... forgot how much i paid tho...