You can buy them used. It's not too hard, you just have to have the money. Something like this one would probably only run you about $5 million. If you can afford that and the ungodly amount of maintenance it takes to keep a fighter jet running, it's completely legal as long as the pilot has the ratings and you obey FAA rules. If you want to arm the sucker, that's a whole different thing. When they sell these privately, they remove all the weapons systems (electronics, and mounting hardware). To get it firing again would take a lot of black market and less-than-legal work done. But with enough money, and the right knowledge of military surplus sales, you could do it.

As a Navy Logostics Officer, my Father worked with Navy JAG a couple decades back on a case where some wealthy rancher out in Montana assembled from surplus parts a full functional Marine Cobra Attack Helicopter, complete with guns and rockets. he used it in his spare time to hunt wolves on his massive piece of property to protect his cattle herds. Needless to say, the government politely asked him to stop...