I happen to have 3 back bumper lowed unpainted plastic parts.
One is on the car
The other 2 I bought for a twin-exhaust project, the plan is to combine the exhaust slots from both bumpers into one. I am almost ready to do this, when finished, I wont need the original back bumper plastic piece that's on my car. If you can wait 3-4 weeks I would be happy to sell it to you. I paid a breaker £70 for the lower plastic piece.
Although there are potentially better deals out there, check out ebay there's a complete one on there current bid £20, you can always throw away the top painted part.