anyone know where I can get sprites on aliens?

1. Google

2. Threads at the top of the Visual Arts Forum. IE: "Sprites, sprites and more sprites!".

3. Make your own, you can find various tutorials in the stickies in the Visual Arts forum, at Pixel Joint, and plenty of other places, along with lots of good examples of other peoples work. (A good tutorial being "So you want to be a pixel artist?")

4. Goto the Help Wanted section here at Gamedev or various other pixel art forums (like Pixel Joint) and try to find an artist who will make them for you, for free or for pay, or find someone kind enough to direct you to more resources.