Here:
Car Side Blindspot Blind Spot Mirror Wide Angle View | eBay
But I modified it by hacking off part of it and then epoxying on a plate and then epoxied that to the window.
Noticed tonight that the mirror had popped loose of the two-sided tape it gets held onto the rotating gimble with. I will take it back out and remove the tape and use epoxy there as well.
I may also fill the section of window forward of the mirror with something like a silicon caulk, just to prevent a small inlet there that might cause some turbulence.
I have another, different style, mirror I will be adding on the drives side. It is smaller than stock, but not this small. It will get a small blind spot dome on it also. I'll post pics when I get that done.