Pork shoulder or pork butt are best for making this dish in the crock pot. Pork shoulder has a good amount of fat that will render down into the sauce, and it is mostly free of sinew and cartilage that would be unpleasant to eat. It is  readily available and very inexpensive. You can buy large pieces of pork shoulder with the bone intact or boneless pieces that are ready to place right in the crock pot. If you only find a large piece still on the bone, you can remove the bone and cut the roast into a few large pieces.