Can anyone tell me where I can get disc slotted rotary encoder? I just searched on the net but still didn't find yet.
There is a number of encoder printing programs (and at least one.eps file for the sam purpose) floating around the net.
If you print one of these and glue it to a thin piece of aluminum plate, you can cut it out yourself.
You might be able to position the IR TX/RX well enough to use a coarse gear wheel as well.
If you take a stepper motor or a synchronous (mains) motor apart you should find a "comb wheel" that can be used either with IR or with a Hall sensor.
How many segments/holes do you need?