Cheap optical rotary encoder - do they exist?

I want to stick an optical encoder on the back of my wheelchair motor. It has a 8mm shaft. I can make adapters if I can find something close.

Anyway. I searched around a little bit and found overpriced expensive ones $39, $20 for example - I am sure they are fine device. I'm really too cheap - this is nothing more than a plastic disk, printed or slotted, with a pair of optical leds. Has anyone found them pre-made - for cheap ? If not, I will just hack apart an old ball mouse and use the guts of it.-

  • (edited) *

Literally 60 seconds after I posted, I finally found what I was looking for after searching for hours. Here they are by the way, in case anyone else was interested:

Fairly priced optical encoder kit - diy