1972 coupe looks like I have an issue with rust on the drivers side metal support located under drivers seat floor pan. not sure where  to find replacement this piece is riveted an screwed to the foor board and has the nut for one of the seat support bolts?

Try Paragon...they have a ton of stuff like that. Might have to give them a call, or e-mail, if it's not listed in the catalog/web.

Thanks

mid america has complete floor boards, left or right sides about 250.00 ea side, mine are also rusted, probably get them replaced this spring. I was

This is true on the later, steel floored cars. They rust pretty bad.(good?)
This question concerned a '72 car, and the steel supports that are riveted under the floor. These supports/backing plates are for attaching the seat belt bolts, the seats themselves, the battery, etc. These are replacable by simply removing the rivets, and installing with either rivets, or bolting them in.
On the later, catalytic converter equipped cars, the floor pans themselves do have to be cut out, and welded in. Sounds like fun, no? They put the steel floor pans in in an attempt to keep some of the heat from the converters out of the cabin.

How do you attach  new rivets? where can I get them? you are right these were supports for the seat belt bolt.

Paragon has the rivets, and I'm sure others may have them also.
The rivets were set at the factory with two tools...one on either side of the part. One side was held with a HEAVY back-up tool, and the other was hammered with a special shaped air-hammer bit. I think Paragon might also offer the tools for some of the different rivets on the car(there are MANY!)...
Otherwise, you just have to be creative...