what do you call the plastic that the electric window motor mounts to?

Don't know which door you're working on but eBay got the whole 'drivers door' assembly for $59.00 .. That's the plastic piece (motor support) the window regulator and the motor! .. Otherwise you need to go to a salvage yard to get the piece you need .. They most likely are not going to sell just the window motor support by itself .. Another method is bolting the window motor in place but you need to have the 'talent' necessary to do that! .. These are just a few suggestions .. Good luck with it!