I have used sulphuric acid and hydrochloric acid for electro etching and both work well on copper and brass. I have not used it on steel (never had the need). Hydrochloric acid can be purchased at swimming pool supply stores or large hardware stores, though I've found the storing and diluting of this particular acid to be a pain. A simpler way to get it in a more usable form is to buy a pickle called "Sparex", available at jewelry supply stores or one-line. It comes as a powder you just mix with distilled water in the amounts you want to use.
If you can't find a good electrolyte suggestion on this forum, try the knifemakers forums. They're always talking about electro-etching their knives with makers marks. As most shop knives are made of carbon steel, whatever they are using would probably be perfect for you.
Here's a link:
http://www.knifeforums.com/ubbthreads/ubbthreads.php