Personally I don't eat much meat and hardly any beef, but if you do there are a lot of good reasons to seek out grass fed beef, from nutrition to ecology to sustainability.
From Whole Foods here's an article on how to cook grass fed beef and why it's better environmentally, biologically, ethically and nutritionally than the grain fed kind.
More places to buy grass fed beef:
American Alps Ranch (Skagit Valley, Washington)
Northwest Grassfed Beef (sourced from Washington, Oregon, Idaho, Montana)
Eatwild/Washington (Grass fed meat, eggs and dairy in WA state)
A resource for grassfed meat, eggs and dairy products, searchable by state
Rain Crow Ranch (Missouri)
Tallgrass Beef (Illinois)
San Rafael Valley Grass-Fed Beef (Arizona)