Flakes of flint broken off a spall in making flint artifacts are excellent, as they usually already have a sharp edge and are of good size. The edge does not need to be razor sharp, but should be at least as sharp as a gunflint. The edge can be sharpened using either pressure or percussion flaking, just as the flint in the flintlock is treated. It needs to be large enough to stick some fingers in for a good grip, again for control. Any correctly tempered high-carbon steel will work.
Old files make excellent steels, and are frequently used for such.
Char Simply put, the traditional char is cloth that has been made into charcoal. It is heated at high temperature in the absence of oxygen to drive off flammable solids in the form of gas, leaving a black cloth which catches and holds a spark, smoldering with a hot ember rather than flaming. Making char cloth is not difficult. All you need is a can that won't melt in the fire, and some cloth. I use a steel 35mm film canister, but smaller and larger ones will work quite well, anything from a lozenge box to a paint can.