FIAT can provide doors, but the cost of a pair painted and fitted is likely to cost more than your Panda did.
Repairs are either a bodge which will bubble through again quite quickly, or a lot of work cutting out rusty metal - and even then it may bubble through further along the door bottom (ask PandamanPaul about that).
There are some doors about off scrapped ones which haven't rusted through yet, but you have to be in the right place at the right time to find them, and even luckier to get them in the correct colour.