I’m a big fan of paper miniatures, and I just found two more sources to share.
Printable Heroes has a ton of different minis. Cartoonish but fun!
Paper Friends from Seven Wonders in the UK also has a lot of minis, including superheroes. My favorites are the ones from 2000 AD though. They have a lot of science fiction sets, including, Dr. Who, Star Trek, Star Wars, Terminator, Predator, Firefly, Planet of the Apes, and Starship Troopers. They also have a bunch on different gaming genres (fantasy, sci fi, horror, spies, pulps, post-apocalypse, and even the Chronicles of Gor!).
PS: Back in 1979, I played a game of Starguard, an early science fiction wargame. I still have a fond spot in my heart for it, so I was delighted to find a page filled with all the miniatures for the game, but also the rules. The name of the place is Tin Soldier. The minis were from Mcewan Miniatures in Salt Lake City. Cool.