You can contact US Digital, they should be able to sell you just the disc based on the ID and OD of the part.
If you take the encoder off and flip it over, there is a good chance the it has a number such as 500 on the bottom side.
You could also get a amt-102 encoder that is capacitive vs optical, has adjustable resolution and is about the same price as the disc you need.
They are available from Digi-key. I was given great service when I purchased mine and it is running really nice on my milling machine. When I get the time I will swap the other 2.