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They don't sell just the plastic to my knowledge. If you wanted just the plastic, it may work to buy a mirror with the same housing but no dimming glass (if they make such a thing) and switch your mirror guts into that housing since that mirror should be cheaper.
There was a thread about making the bed rail caps look black again, maybe one of those plastic treatments would darken your mirror again for at least awhile.
On my previous suv I had the mirrors that were the same type of black abs plastic as the ones on our trucks and I actually took some black spraypaint and repainted them and they looked good as new. Duplicolor makes some "Trim / Bumper" paint for just these types of thing if interested.