How bad is the damage?
If it is just bent You could remove it and either straighten it yourself, of have a metal shop do it for you. If it is cracked/broken they could also MIG weld it for you.
The only way I know to get one of the ribs is a boneyard or someone with NOS. If you can find a section of rib that will span the damage in he current rib the manual says to rivet it on the back with a 6 inch overlap using bucked rivets. I would assume olympics would work.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato