Joe,
If you have a color inkjet printer and some ideas, there are sources for the water-slide decal backing film. When you get the inspiration pretty well organized, doodle it out and print it on that film.
The decal film needs to be sealed after you print on it so water doesn't dissolve your image.
If you can get some images scanned into a file your word processor can load, you can use those. MS Word is pretty much universal, now, and it has a thing called WordArt that lets you do all kind of neat things with text. Most likely you already have it. Other graphic programs are around, too.
There is a bit more to it than that, but not really much that's a problem. And, you wind up with something you came up with - you're not stuck with what happens to be available where you find it.
Done right, these waterslide decals can be thinner than any sign-shop cut vinyl stick on - almost invisible edges - and more intricate than you'd ever agree to pay someone to cut for you.
Plenty of guys in here have messed with home-made water slide decals - you may hear from them...