Try the junkyard or the wreckers for replacement panels.
Otherwise, you will have to go down the route of fixing them... Which is NOT easy because your going to have to sand and grind off the rust. Then rebuild the panel from scratch using Bondo (easiest way), or cutting out sheet metal and welding on new peices. No this isn't anykind of normal welding because you will need a special Acetylene, and O2 mix. With a special type of metal alloy to prevent the new part from rusting. Then you'd need certified training etc... to use one of those damn things...
But you can always try a mig welder to do the job
Body work is a lot of work, and I can't even begin to tell you what more is involved.
So I hope that helps, any more further questions or concerns, don't hesitate to PM me.