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Not sure about finding a used one for sale, but I'm pretty sure this is what you're looking for. Quite expensive for what it is. Mine is missing too and needs to be replaced. In case the link breaks, the part number 'should' be... ERR7229 (I've seen this rubber plug called many different things). If this isn't what you're looking for, I'll remove this reply (so I don't confuse anyone).
This is the hole I was referring to for the rubber plug (ERR7229) - see second picture. I'm not sure what's under that metal plate (if there's a rubber plug too) as I've never removed that - see first picture. And for reference... The metal plate (you're referring to - and in my first picture) is horizontal to the ground, the rubber plug (ERR7229) that I'm referring to (in second picture) gets installed vertical to the ground (on the engine side of that housing).
I need to check I might have one available, if not I can get one pretty quick as it should be on my "new" spare transmission if it missing the seller will ship it to me