You mean
46 509-23148-L0-00 CASE, DUST SEAL DISCONTINUED?
From yamaha-motor.com:
From memory, the dust seals were pretty generic, and I'd be surprised if you couldn't find something from a YZ125 or XT/TT500 that did the job.
The air/oil seals themselves are still available, but DJRacing is your man with the info - he found some when I failed.