On play station 1, 2 or 3, there isn't so far as any ones discovered but you can download a UFO mod if yours is PC/any other computer maybe mac, at cheatgarage.com
you must mod the gamehow u mod it?
There are no cheats to get an UFO, you need to have an mod for that.
cheat or find it at area 51 or whatever number it is
you cant without hacking