My first suggestion would be The Butcher, up near UBC (on W 10 a little bit down from the Safeway) I'm pretty sure they have grass-fed meat.
Jackson's Meat and Deli (on W 4 2200 block, and I think there's also a Granville Island location)
There are actually a couple of butcher shops on W 4 near Jackson's, I'd check all of those out.
The obvious, Whole Foods or Choices market.
Greens Organic Market at Broadway and Maple has grass-fed meat, I think, though I don't actually shop there so I'm not certain.
Stongs (around W 30th and Dunbar) has a good meat department for a grocery store and it's much cheaper than Whole Foods.
Seconding the suggestion to go ask at the butcher shops on Granville Island
There's also this place opening up on West Broadway soon.