I cannot find replacements for a "special" bath (and sink) tap washers. I do not know the manufacturer of the taps (its not on them anywhere, I was not the installer).
The washer is a variant of the 3/4 inch washer size but on the side that interfaces with the base of the tap is slightly (and crucially) bigger.
The tap jumper consists of a little shallow pan holder that the washer sits in (fixed with a screw) and sits on the edge of the sides. The washer is irregular in that it has an instep so the side that fits in its holder is
3/4 inch and the side that sits outside is slightly bigger by about 1/16 of an inch. Therefore the trick of turning over an old washer can't be done.
I have tried using various standard 3/4 inch washers (which sit in the little pan) but the tap always drips and so is no good. Therefore I think the overhang matters.
Any ideas? Regards, Roy