Helen, the following website has some nuts and bolts about this subject at....
http://www.realmilk.com/cowfarmshare.html
I hope you can pull this off in your area.
Don't look for the cow with the most milk, get one with the "best" milk. Brown Swiss would have a much higher chance of having A2 milk than a Holstein. (google A2 milk for why it should be our "milk" of choice.
Feed it the cleanest minerals you can find (Redmond Salt, an Ancient sequestered source, before man's impurities) and good grass and hay (the higher the brix level the better).
Steve