I made my wool dryer balls from scrap wool yarn. You just wind up a ball to right size and drop it in the leg of a pantyhose and then tie a knot and drop another one in until you fill up the leg. I washed them in the washer and laid them outside to dry. The wool felted nicely and has not unraveled. You can quite often buy wool yarn cheap at the 2nd hand stores. Don't use washable yarn as it will not felt.