Very common problem on all Metros, especially where road salt is used. Patching it might work if it's not too far gone. You'll have to take it off and see. It has a plastic coating, that once breached, traps moisture and allows rust to thrive, so it can be worse than it looks at first.
fillnernecksupply.com has new ones, but their webstore appears to be down. They're about $90 though.
You could go junkyard hunting and hope you find a rust-free one.
Or you could make one:
http://geometroforum.com/topic/3546230
They're not too hard to change. You should be able to change it with gas in the tank, depending on how much there is. How full is it? Having the rear jacked up will help. I'm thinking 3/4 tank or less would be ideal.