There was a member here Knucklebusted that made some 3D printed ones. I bought a set and look good just havent gotten around to instilling them yet... try searching 3D sport mirror gaskets of post from him.
I have the ones that TPP reproduces if you can't find nice originals...… (and any hardware you may need . . . . . .) $8.00 for a Pair.
As long as the are wedge-shaped,and not flat,you are good. I also found some mid-70’s Cutlass mirror gaskets will work. They are wedge-shaped,and we’re made for the same type of sport mirror. NOT correct in appearance,but the mirror will install at the correct angle.