If you get all the correct licenses, certifications, permits and whatnot and have very sufficient funds (because maintenance alone will render you bankrupt unless you've got Midas' hands) you can get pretty much any vehicle save for Naval Vessels which I've never heard of anyone acquiring.

And as ObadiahtheSlim said, The availability of the modern stuff to private entities and persons is near-nonexistent unless you're some sort of aggressor pilot who's contract-bound to 5000 different things by the Air Force. So you're stuck with surplus/decommissioned stuff.

It's not like I can just call McDonnell-Douglas and order an F-15C and munitions.

Because I didn't identify as an attack helicopter.