This is probably not as useful of an answer you're looking for, but I figured it was at least helpful... I have been on the same hunt!
Kelona SuperNatural claims grass fed, but I've only seen their yogurt/cream here in Cali (but milk/cream in other states). Might be worth checking into or asking stores that carry their other products: http://www.kalonasupernatural.com/our-process/
Organic Pastures (http://www.organicpastures.com/) also claims grass fed, but I believe it's "mostly" grass fed... so I am not positive if that's 100% year round. Andronico's carries them, and I've seen it at farmers markets. Also: raw, not cheap.
Ditto Claravale, which makes a statement for cows not being pastured year round here: http://claravaledairy.com/faq.html
Clover has cows pastured half the year: http://www.cloverorganicfarms.com/Ask_Clo.asp
I haven't seen/tried this before, but Humboldt Creamery says they "graze year-round in lush, open green pastures": http://www.fosterfarmsdairy.com/?p=175