rails are a stock item at most RV shops. They come in 2 sizes so get the one to match your awning
. They also come in different colours (aluminum and white).
Some people are attaching them with VHB tape rather than rivets. Search here for awnings and tracks before making a decision. The 4' are meant to easily ship, the 8' is standard, they come longer.
Somewhere here there is a thread that shows pictures of various awning lengths, brands and tracks on our trailers. Sorry I don't have any time left this morning to search for you.