Pulled pork comes from smoking a pork shoulder. Often times, you will hear this referred to as "pork butt". That is also correct. When you buy a pork shoulder, it will come as two pieces. One of the pieces is the Boston butt, and the other is the Picnic. Not sure where the names came from.
When you buy a pork shoulder, they are available as boneless or bone-in. I use either, depending on what's available. Some prefer the bone-in, and say that the meat is more flavorful. I have not been able to tell the difference.
I usually buy meat at Costco. They have great pork shoulders, ribs, and brisket.
Most of the shoulders are between 14 - 18 pounds.