any exhaust shop.. they should keep stock of sizes smaller then 3"...
copper spray gasket would be ideal for hot exhaust parts....
As long as mounting flanges are ture and clean you shouldn't need anything. Don't use RTV on exhaust manifold or before o2 sensor. Probably wouldn't affect this age cars but newer cars o2 sensors get affected by the sealant curing and the fumes/particles coat the o2 sensor causing it to give false readings.
as per above, plus RTV does go hard after a while.. so long as your using a new 2 layer stainless gasket for the manifold, and everything is clean, you can use copper spray to help it seal better.. other wise not really needed, but good just in case kinda thing....