Lemon juice will work as will vinegar (warmed up) I would be careful with the vinegar, I have used it before, and if you are not paying attention, little bubbles form on the blade and give a polka-dotted etch. I am guessing it is not the look you are going for. With vinegar, just move the knife in the vinegar every few minutes to prevent bubbles from forming on the blade. Ferric is your best bet if you can find it. Radio shack still has some in a pcb etching kit, but it is kind of expensive to buy the whole kit for the little bottle of ferric. I think it is like $20, and you get a fairly small bottle of ferric, which is ok, because you should dilute the ferric about 20:1, and I have heard some people say as much as 50:1. Hope this helps.