One can use old T shirts. When you can pick up a bag of them for 2 or 3 dollars, you can change them out as they get ratty, for very little investment.
I wouldn't use Sunbrella, though, because it has little chafe resistance.
I have used, also,
training pants legs from thrift shops--I like the grey ones. I have bought that kind of
fabric and made fender covers out of it. They last okay, but are vulnerable to snagging on sticking out nails, and the ilk. Levi's legs would also
work pretty well. The trick is to find the jeans in the thrift shops that are large enough diameter. Our fenders are pretty big.
The most recent ones I've made were from some polyester "canvas" I found in a yard goods store, on
sale. They have faded, but are still in good condition after about 5 years....however, our usage of them is only occasional, as we
anchor out far more often than we go into
marinas.
I think the stuff
sold by
Sailrite is probably the best
fabric for the purpose, and if you use ptfe thread to sew them, they'll last quite well, but it will depend on your use and how strong the UV is where you are.
Ann