That sounds like a standard Dowty washer? A flat metal ring with a Dowty rubber seal bonded into the centre? Maybe your rubber has come loose? Usually they're silver/grey or yellow chromate colour. Not sure that they can be copper, I suspect aluminium or steel is more likely.
The size you need might be 22ID x 27OD x 2.5mm. You should be able to get them from any place that does hydraulic or fluid handling stuff or a decent motor factor. Maybe even a plumbing place, or ebay or Amazon.
The GM Dowty part might have been superseded by a copper washer. Or it might be that the copper is superseded by the Dowty, the part numbers appear to be the same. Whatever, I'd use the same sort of washers you started with.
The Dowty washers require a lot less torque than plain copper, so don't over-tighten.
GM part nr: 24427589 aka production nr: 11079631.