It sounds as though you're talking about ninhydrin. It's a purple dye that reacts with human sweat and turns bright purple. The best place to ask is at your local police station. They can usually tell where to get some.
Good advice... but why do you want some?
I think they are just using some basic ink. But they do have a system where the ink runs out when someone tries to open the case wrongly.
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Other people asked questions on various topics, and are still waiting for answer. Would be great if you can take a sec and answer them