I print my own decals, It's easy, and works well.
1.) You will need a computer, some kind of graphics program, and an inkjet printer.
2.) You will need
inkjet printer transfer sheets in either white or clear, as well as a can of high-gloss spray paint. The glossier the better.
3.) Go ahead and design your transfers in your graphics program, or
download them from somewhere. Remember that inkjet printers do not print white, they leave white sections clear. If you use white paper, you'll need to trim each detail carefully. There is a
better tutorial for doing this here.
4.) There is a great tutorial
here for actually applying them.