I posted a "wanted" ad on pinside for one in the last month or so. Someone did reply and sold me the whole assembly (and he even mentioned how he kept coming across broken ones))... and I was super grateful!... but it wasn't the cheapest thing. I am almost afraid to install it (the original waited 40+ years for me to show up and break it!) There seems to be a lot of Grand Prix owners with the same broken drop target - it is that angle and the speed of the ball at that close range I suppose.
If there was ever a candidate part for a vendor to make an aftermarket version, this is it. Hell, they made over 10,000 Grand Prix units. I have a Spin Out that I can buy new plastics for and they made less than 3,000 units. I feel like there is someone with a 3d printer somewhere than can help us. Makes me wish I had a few backups