For the front Ed is spot on. For the rears it's a little more complicated.
I am not really sure but I think that up to '73 the RSR rear fenders were 11' wide. To accomplish this you could get turbo style flares. There are actually 2 different styles.. early turbo (they have a complete round hole for the torsion bar)
or late style (half moon hole for t-bar).
With '74 I think the RS (or IROC) had 11' wide rear flares. The RSR went up to '13 wide flares. If you want these you either go fiberglass or you need to make them as Martin was saying.
I think Pelican Parts still carries the turbo style flares. If not try
http://www.restoration-design.com/
http://www.mamotorworks.com/
http://www.tweeks.com/