I grab any older and well-worn wash cloths and those smaller hand towels that are around the house. Nice and soft, largely lint free, and when they get a little dusty, toss 'em in the wash. Also, really handy to have for a quick wipe down after a little rain shower, or for perspiration, or whatever. My bag's got two or three of them in it all the time.
If one gets a hole or a tear, I then put it in the garage to use there, because they shed threads like crazy.