If you Google the awning
manufacturer and number and 'replacement awning fabric' you should get lots of hits.
In my case, I got a good price and availability from :
RV Parts, RV Accessories RV Supplies Priced Low
I haven't ordered it yet as they don't want you to store it boxed up and I'm not yet ready to unfold it and install it.
Instructions for measuring and installing are on the Net. Be aware that there is some hazard in the installation, as the awning is rolled up with powerful springs that could cause some damage if released accidentally. Use care.