Can I buy GTA money?

I'm curious, is it possible to purchase in-game currency for Grand Theft Auto, also known as GTA money? I've been enjoying the game but have found myself in need of some extra cash to buy some of the more expensive items and upgrades. However, I'm unsure if this is something that's allowed or even possible. Can you shed some light on the matter and let me know if there's a legitimate way to acquire GTA money?