I just buy tea stained muslin. Very cheap 100% cotton fabric. Sold everywhere that sewing materials are sold. Just nick the edge at the size you want and tear.
I never hem it. unraveling is not much of a problem. When it gets to "ratty" looking, I trash it and make another.
I usually carry several. A small one for the occasional rag. A medium one I use as a towel, sit on, or strain junk from water. A large one I use as a shawl or head cover in the desert. Wet it and cover your head. Almost too cold...
Very useful item.
Nomad.
_________________________
...........From Nomad.........Been "on the road" since '97