The front header (which the fabric attaches to) of my '68 convertible top frame is rusted out. Can this piece be replaced without replacing the entire convertible frame? If so, where can I find one?

 

The top header bow is replaceable. I personally have never done it, so I don't know what it involves to do it. Rick's Camaro carries it, so I would think other suppliers would also have it too. For some reason, I cannot copy and paste a link to Rick's catalog page. You'll find it listed under convertible tops in the exterior parts and trim section.
Part #: 33-182840-1 Price: $245.99 ea.