I spent some time looking and didn't find anything.
According to Wikipedia, the Plymouth Belvedere, Plymouth Savoy, Plymouth Satellite, Dodge Charger, Plymouth GTX and Plymouth Road Runner are related. Doesn't mean the parts will match but you might get lucky on one of them. Sounds like the Plymouth Satellite might be the best bet.
AutoMetal Direct might carry fenders for some of them. I'd also broaden the search for '71 and '72 models as that's the introduction of the new body style.
You can set up a search on Ebay and hope that one will show up.
I'd also use one of the parts services that puts out requests to junk yards.
You might as well pull it off and see what else you'll need. Probably the inner fender shield, trim, etc.
Lets see some pics.
Here's a 73 Plymouth Satellite fender for a two door on Ebay. Pricey buggers.
Here's one for a four door.