There are two main types of pork shoulder, the Boston Butt (upper) and the Picnic (lower). The Boston Butt is the preferred of the two. You're going to want to ask for a Boston Butt and your butcher will know what you want.
I just warmed some up tonight that I had pulled out of my freezer. Was so good, I just ate it plain.
Check out this pic that shows the locations of the two shoulder pieces:
http://0.tqn.com/d/culinaryarts/1/0/d/H/-/-/cutsofpork.jpg
Its not the best picture. But it gives you an idea of where the roasts are located.