http://www.motosport.com/motorcycle/KEITI-GAS-CAP-PAD-SUZUKI-BLUE-FLAME-2002?ride=SUZUKI;GS500E;1994
pretty good item it seems for so cheap.
Def. beats me having to repaint my gas cap.
Going to browse around a bit more and see if I can find anything other then blue and white.
You could get ahold of some adhesive backed vinyl in any pattern you want, and just cut it to fit.
Something like this:
http://www.vinylforcricut.com/servlet/the-265/Purple-Glossy-Adhesive-Backed/Detail
Or find someone with a vinyl printer (sticker shops, t-shirts) and slip them a few bucks to run your design through their printer.
Don't you have any leftover paint?