The drain plug washer used by Honda on our bikes, (the 1000, 1100 & 1200), is also used for many Honda car models. If you go to this site ->
<- and enter in the part # 94109-14000, then drill down to the info you will see all of the car models that use this same part. I have found that many o-rings and washers from the bikes can be cross-referenced to a car application. So I imagine that you can get a OEM washer from a local Honda dealer, or at least you can go to NAPA and get one for one of the cars listed.
'85 GL1200 Interstate