Many vendors are selling what they call tow. Tow by the first definition is the coarse fibers left after the processing of flax into linen threads. Tow from flax is quite coarse and is an excellent cleaner of fouling from black powder. It can rinsed and reused. After that tow becomes any fiber that looks like tow resulting from the processing of flax into linen. The good news is that these other forms of tow are still pretty good. Dixie Gun Works, Log Cabin Shop, Track of the Wolf and other vendors have tow made from jute, hemp, manila fibers or sisal. You can go to most hardware stores or hobby shops and buy twine from jute, hemp, manila or sisal. Cut the twine into 4" long lengths and unravel into single strands. This can be used in any application that calls for the use of tow. As Mike Beliveau shows, tow can be used as a wad for holding a ball in a smooth bore. It will work with shot too but does not seal the bore as well as wads from other materials. Tow can used a scrubbing cleaner for your firearm. I have a wad of tow tied with a long linen string that I use to wipe my bore between shots. The coarse nature of tow will pick up the fouling in the strands and not push as much down the barrel into the firing chamber. Since the tow can be rinsed of the accumulated fouling it can be reused quite a few times. As
@Loyalist Dave suggests, this well used and rinsed (and dried) tow is an excellent tinder for fire starting.
The best place to get tow is from someone who processes flax into linen. These sources are hard to find. Since the other sources for tow are acceptable, get or make your own tow from twine.