If you're looking to buy on-line, and are in the US, I'd suggest looking at the Valley Yarn line at Webs.com . I particularly love the softness of the Berkshire, though they are all a nice wool for good value. Webs often has sales on various yarns. Littleknits.com is also a good site for great sales on great wools.
A lot depends on how much you're willing to spend. I agree that the Rowan yarns, especially from a store, would make a very expensive blanket.
I'm sure you'll get lots of favorites--Cascade 220; Plymouth Encore for a machine-washable blend, etc.