I'm buying from slankers:
http://www.texasgrassfedbeef.com/id78.htmthey say they do not feed any grains (including corn).
" Most of what they eat is grass, forbes, and leaves of trees. But they also eat grubs, roots, acorns, berries, fruits (acorns, berries, and fruits are strickly seasonal), eggs, critters of all kinds, and about anything else they come across in the pastures and woods where they live."
if you look over the website, the supplier (ted slanker) seems to highly value truly grass-fed meat, and makes a big deal about it online. He prescribes to a cooked paleo diet.
Also the beef and bison I have ordered from him have been the best tasting I have tried other than fresh. So I have reason to believe his pigs really are on the diet he says.