Honestly, I think you can use just about any automotive primer from an auto store that's rust colored. I've got both Duplicolor and Rustoleum rattle cans in my cabinet, both of which I'd be hard pressed to tell apart were it not that one was white and the other is grey.
Be sure to buy automotive primer, however. I've got a can of flat black enamel that I've used as primer before. Works fine on metal minis, but it stayed tacky to the touch on my Reaper Bones (pvc like material). Can't recall exactly what it was (acetone perhaps), but there's a chemical in spray enamel that's not in automotive primer, and that chemical doesn't play well with PVC. Not sure if it would have the same effect on regular GW plastic or any resin, but I personally am staying away from using it with those.
