It's called a 'weathershield'. Usually sold as part of the whole awning (most typically on acrylic awnings, not usually on vinyl).
If your current awning is Dometic (A&E) brand, give the manufacturer a call, as I think they'll deal directly with you. If nothing else can answer questions. 1-800-544-4881
I think with Carefree, you'd have to go through a dealer for parts. (However, you may be able to use this on whatever brand you have, so definitely ask).
Another avenue is to try an RV salvage yard. They may have one lying around that you could pick up if you live near to one.
Hope this helps!