Those were beautiful cars and really very nice. Fast, comfortable, handled well. My BIL had one way back in 62 or so. Always liked the 57’s and 58’s. I didn’t know they had any worse rust problems than the rest of the cars back then but maybe. They really didn’t do any rust prevention and lots of places that held water and salt and promoted rust like the holes for the clips for the trim all over.
I don’t know where you are but you might try calling Denver Bumper. They do restoration on old bumpers and are pretty big and good. I’m not sure if they stock them or not but you have a gem and they might be able to refer you if they don’t buy them. Just do a google on them.