Mepro has some that they advertise on shotguns. If you really want to spend money some LE mags show the Aimpoint M4 on a shotgun. And I believe Trijicon now has a few that I have seen advertised on a shotgun.
Good point, BBBUUUUTTTT if you are really dead set on an optic EoTech makes a nice shotgun opticGet a tritium bead instead. You'll need to spend at least a few hundred for an OK optic that will stand up to abuse...a tritium bead costs like $20. It'll be nice and bright, won't add any weight to the gun and means you don't have anything extra sticking out to get caught on something.
More money for ammo and training, too.