The most information I have found on them has been on the radio controlled vehicle hobby sites and model car sites.
But this is one of the guys that got me interested - he does not have any for sale right now on his Etsy site - but he has an amazing tutorial:
This pic on Flikr shows some decals applied too:
This guy on Hisstank has made quite a few:
Hope this helps to get you started, if you find a source that makes Joe Decals for a fair price, please let the board know.