Actually, "aqua fortis" is the Latin name for nitric acid. The name means "strong water". What we use for stock staining is made with aqua fortis (nitric acid), but it is, technically, not aqua fortis. As noted by others, it is a solution of iron (ferric) nitrate. Ferric nitrate solutions are also used in the jewelry trade to etch silver with decorative designs. If worse comes to worse, dilute nitric acid is fairly easy to make with hydrochloric acid (i.e. muriatic acid), some water, and a nitrate salt (potassium nitrate, sodium nitrate, or ammonium nitrate). With the dilute nitric acid, you can dissolve some iron and make the iron nitrate solution you need. Better living through chemistry......