Where can I get this liquid/dye?

Question:I heard that there is a liquid or dye that can be put onto banknotes which leaves a long lasting green colour (or another colour maybe) on anyone who handles (as in steals) the said banknotes, Anyone know what its called and where I could get it in the UK?

Answers:
You need to check to see if this is legal. The only people I know who do this are the police departments..