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:
http://insidiouscustoms.blogspot.com/2010/04/how-to-apply-custom-made-waterslide.html
This pic on Flikr shows some decals applied too:
http://www.flickr.com/photos/28783781@N06/5475215571/
This guy on Hisstank has made quite a few:
http://www.hisstank.com/forum/g-i-joe-customs-finished-projects/119095-first-try-waterslide-decals-ceraurus.html
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.