You can find the parts list in the service manual at
https://mans.io/files/viewer/427728/46#navigate_bar.
It looks like the part you want is a CAR AUDIO ENCODER WITH PUSH PIONEER VOLUME SEARCH TRACK. Pioneer Part #CSD1168. The Pioneer site refers you to this supplier, but you can find a few others as well
https://www.pacparts.com/part.cfm?part_no=CSD1168&mfg=Pioneer
You can search rotary encoders at Digikey but the push button may make it hard to find a 3rd party replacement.