Sensor issues, help?

[CodeMaster 1388]

Ok so originally we were going to use pots to get the location of some of our components as they moved, unfortunately, the shafts have been built in a way that makes this impossible. If you've seen the most recent image of our robot, you can see that there're metal disks on one of the shafts lower on the bot. We have the idea of marking the disk, and using an optical rotation sensor in order to read how many lines have past by, and thus the position of the component. The only problem is I can't find anywhere to buy an optical rotation sensor. The exact kind of sensor I'm talking about can be found at this site, under Optical Rotation Sensor.

http://www.cyanogen.com/help/maxdome...Controller.htm

If you know anywhere we can find these, please post it here, I'm having no luck.