It's pretty easy to accesorize a standard AR-15 for lefties. Most rifles already come with a shell deflector, and things like ambi selectors and charging handled are simple drop in parts. There are even single sided safeties that can be swapped over to the opposite side of the rifle for lefty operation, making things even easier.
If you really want a gun that's lefty friendly there are a few companies out there making lowers with fully ambidextrous controls, allowing you to operate the mag release and the bolt release from either side of the rifle, but they usually aren't very cheap.
"Si vis pacem, para bellum" - If you want peace, prepare for war.