Very simply, tool steel is used in making many types of tools. The best tool steel can be found in old files, letter or number punches, hacksaw blades or knives. Now, a word to the wise, fellers. If you use your wife's brand new mill bastard for this process you will have more sparks in more places than you care for. She will not be a happy camper.
Old files work well, too, - just be sure they are past their prime.
The next most important thing, if you're making your own striker, is to remember to temper it when you're through shaping it. This just means to heat it to a cherry red color and quench it in brine or diesel oil. If you don't want to make a striker, you can buy one from traders or catalogs for around ten dollars. A good striker will be made of high carbon tool steel with a dull knife edge with which to strike the flint. Once you have obtained the striker, it is time to go in search of flint.