A got adventurous a while back, and bought a 5 image a month subscription to Fotilia.com (owned by Adobe). It's a stock image site with a more diverse than normal selection of material (some great sci-fi images). However, the end of the year is approaching, and I have an extra 7.5 images, with no more covers to design.
If anyone needs new covers, feel free to search Folila for images. If you find any, let me know and I'll send them to you (up to my outstanding balance. I don't really need them, because I've already formatted my next eight books! (By the way, it's also a great way to pick up artistic dividers and chapter header graphics.)
If you don't see any images you like, create a free account since they show members additional content. Beyond that, you're on your own.