OP - Sorry to hear that those parts are no longer available - They have been in service a long time. This forum is a great resource for locating parts & service.
Good Luck!
(To those complaining/inferring "ARC won't repair...", please know that the M300 MkII Amps were introduced in 1987. Have your Amps lasted that long? I doubt it...Many companies are still working to resource their parts and accessories from 4 years ago (Pre-COVID).