Yo!

Celtic Crosses
More Celtic Crosses, and other images.
More, Fancy Celtic Crosses
Guess What? You got it...Celtic Crosses.

In fact here's the page I pulled those links off: www.google.com

I know some of the images are small, but once you save the picture (right click on the image, and look for 'Save Picture As'), you can enlarge it, maybe, enough to make it usuable for your purposes.
There's 13 000, odd entries for Celtic Crosses, so, happy hunting.

B.