Where can I buy a jet in GTA 5?

Excuse me, I'm a bit of a novice when it comes to the world of Grand Theft Auto 5. I've been playing the game for a while now and I've been saving up my virtual cash for something really special. I've heard that it's possible to purchase a jet in the game, but I'm not sure where to go to make that happen. Could you please guide me on where I can find a jet for sale in GTA 5? I'd really appreciate any tips or advice you might have on this matter.