And what makes you think they don't know you're masturbating? Everyone does it, even them! Still, you're entitled to privacy.
Somehow, i think you probably have a bit of pocket change, why not just go buy whatever it is you like and keep it to yourself? Remember, that anything on the ingredients label you either cannot pronounce, cannot figure out, or contains any type of alcohol is probably not a good thing to be rubbing into your penis, as it can irritate.