Because of the way it is designed, (hole through frame rail with nut welded to internal plate), they tend to rust really good. It also makes it real hard to get any penetrant into the threads. Like I said in the other post I busted both of mine trying to get them out, after a couple of days of hitting with PB Blaster. As far as I know those bolts are only available one place (AMK). It really sucks drilling them out and tapping after they break.
I have got to hand it to you guys in the east. Years ago I bought a 66 covair tha was from PA. to get the turbo engine. So in removing parts from the underside, every bolt we tried to remove broke. It became so funny that there was a bet if we could get one bolt free. It was a good thing that it was the engine we wanted. Took out the engine and transaxle. I kept the engine and sold the transaxle for what I paid for the car. The entire exhaust system had to be thrown away.