Where is the rust? Top, bottom or sides? Both sides & the roof can be bought but you have to buy the entire panel and it is expensive. You can try to get used parts & make patch panels from them. That would be the easiest way, or fabricate something. A new roof panel is over $300 and the posts are over $200.

1957 Chevrolet 1/2-ton Stepside LB in the Gallery
My Photobucket shots

The person who has nothing for which he is willing to fight, nothing which is more important than his own personal safety, is a miserable creature and has no chance of being free unless made and kept so by the exertions of better men than himself." - John Stuart Mill (1806 - 1873)