I was changing the bulbs in the license plate light holder to LED today when one of the rubber gaskets fell off and behind the license plate area. I probably could remove the rear diffuser to get at that area (and likely will some day). But for the moment I needed a replacement gasket (or could only run one of the two lights).
Not finding a part number for just the gasket, only the whole light holder assembly ( https://www.courtesyparts.com/oem-parts ... 265108991c ) for ~$40, I set out to source a suitably-sized washer.
What I found is that a garden hose washer works well. Maybe not every size of them (some are likely too thick), but most that I found in our collection of them worked fine. If anything the fit is a bit tighter than the stock washer.
The rubber gasket goes between the housing at the bottom of the picture and the middle part, which holds the bulb and 'twist-locks' into the housing.