Where to obtain a tank
You can get a tank (aka, Rhino) in many ways:
- Without wanted level - You can purchase it, and it will spawn in a hangar in the airfield
- With wanted level - You can find one in the military base, but that can be hard to get into. I suggest getting a fast car with Franklin, then using his special abilities to bust through the gate because your wanted level will not go up while you're in the special ability. That will save you some time to get closer before all hell breaks loose.
You can find a tutorial here to get the tank
I'm sure there will also be a cheat soon figured out that would allow you to spawn them, but I don't think it has been confirmed yet.
Bonus: Here is some gameplay footage of the tank