They're just about non-existent anymore. As mentioned, Grand Prix's 78 thru 80 are IMO the best candidate. I've also taken them off 80 Centurys, a station wagon no-less, Pontiac Lemans, same generation. I'm going to put one on my car soon. Stopped selling them 'cause they're going away.
They were on 77 thru 80 BOP's midsize cars. I got mine off of a 79 Olds station wagon. They are out there, you just gotta start hittin' the boneyards to find one........good luck
You might have to slot a couple of holes in the support or add a shim or two, but it's pretty straight forward...........I have found it easier to take the nose of the car off to install it.