Nitric acid (HNO3). In dilute form it will stain (oxidize) many organics, including your skin to brown. In full or strong concentration it is an extremely vigorous, oxidizing acid.
Hazmat laws, fees and liability concerns make it hard to get stuff like this. Bryant Labs in Berkeley CA sold this and worse stuff, delivered -- for very high $ -- but only to "legitimate businesses." My consulting practice qualified (I the purchases up in person and just gave them a card.)
If you know anyone in a plating firm, chem lab, university lab, etc. --they might help you. I wouldn't be surprised if you can order more dilute forms, that will probably do what you need done. Try Google.
