I just got done with a an exercise making some vectored graphics for some decal repros.
If you're reasonably handy with software apps and you like to learn new things, download a trial copy of Adobe Illustrator and make your own graphics. I duplicated the stock SuperDuke graphics by tracing the images in the parts manual and they are perfect. I can resize them and send the .CSV file to a decal shop and they will knock them up in any color and size I specify. A good decal shop will make the stickers with transfer material so they're easy to apply.
If you get good with AI, you can make sum really cool custom images.