is the big site with all things He-Man, cartoon or otherwise.
I know what kind of model sheets you're looking for, and they're hard to find indeed. In the meantime, here's a link to lots of production art:
And this will help especially:
If you hunt around the site, you'll see (and hopefully copy) enough images to get a hang of the style.
I'd check out Bridgeman and Hogarth (drawing books in your library or bookstores), and a bodybuilding magazine or two, and analyze their applications in the He-Man/Masters series. Especially Hogarth; seems to be quite influential in the designs. The Marvel How-To book draws quite a bit from Hogarth.