If you're buying Whole Shoulders, look for them to weigh 16-17lbs. I've found that this size shoulder trims down to the perfect contest size.
When cooking butts I stick with the 8lb range. Stay away from the larger cuts of meat. If you're feeding lots of folks and want the best value, the larger sizes are great, but I want the best piece of meat that I can possibly get for a contest.
Get to know your local butcher. He will have access to the freshest meat, and as you build a working relationship with him, he will be able to keep an eye out for the best cuts that you want. My butcher knows exactly what I want now that I've been working with him for a while. As meat comes in (usually by the case) he puts the prime cuts to the side and holds them for me. For information on finding a good, local butcher... click here.
Stores such as Sams Club or Costco have really good product. It's hard to find whole shoulders at these places, but they do sell really good pork butts.
Normally found in a two pack, the butts are really fresh. Watch out for any "loose" packaging. These have been exposed to air and will not be as fresh. I've found that in the two pack, one of the butts is usually near perfect, but the other may be underweight or have some flaws. This isn't always the case but with a little patience and a trained eye you can find really good meat at a Warehouse store.
The next important item to look for is the muscle on the front of the butt. Often called the "money muscle", this is where prime judging meat is found. As you examine the package, feel the front of the butt (farthest from the bone). It should feel fairly firm. The money muscle should have a decent amount of fat marbling throughout.