One of our favorite butcher shops or "macelleria" is the Luca Menoni stand in the Mercato Sant'Ambrogio (we can't say the favorite because there are so many good ones). They actually have a pretty useful website, in Italian only: http://www.macellerialuca.it
We get all kinds of stuff there, bistecca cavalo, salscicce, polpette, agnello, coniglio - everything has always been top quality and they really go out of their way to make sure you know how to cook what you are buying (and the receipts have printed cooking instructions for whatever you buy - neat). They also have some things ready to cook - like breaded beef cutlets, pork roasts tied with rosemary, etc. And - they have white eggs (good for dying at Easter and hard to find!) and a full selection of poultry and game birds.
Overall we highly recommend it!