The shortage of games probably comes down to the fact that using characters from other games without permission is a pretty cut and dry case of IP violations, which is only made worse if they push an angle that you're also harming their IP's image by using their characters in porn. There'll still probably be some out there, but they'll be few and far between. You'll tend to have better luck finding still images, animation shorts and comics - since they can at least kinda make a defense for it being transformative work that isn't directly competing with the original. Plus that sort of stuff is usually too small fry for them to go after and adds a bit of value in terms of keeping people interested in their franchise. Not to mention it's just bad PR to go after fan artists. But none of that is a sure thing, they can come after you any time they want and ruin your life, so most don't see it being a good idea or worthy time investment.
