AndyMark has some cheapy encoders (
http://store.andymark.biz/am-0174.html). They're kind-of annoying because you're suppose to superglue them to the drive shaft, but they work.
Newark has a ton of less specialized encoders. They run $5-10. Just go to their site and search "encoder" then click on absolute/mechanical.
www.newark.com
Jameco also has half of a dozen choices from $3-20. Try the same search at:
http://www.jameco.com.
I also tried digi-key, but they seem to only sell the components if you wanted to build your own.