Fred Plotkin's suggestions for buying meats in this area of Norcia seem to be spot on. The black truffles of this area are different for spring and autumn. The spring one is though to be superior and is slightly more expensive.
In the main street near the square of St. Benedict you'll find a store specialising in truffle products, and I purchased truffles, and truffle paste here, but I found the oil to be cheaper in Bologna or even Peck Milan!
Make sure if you are there for a very short time that you work out where you want to buy goods and find out there opening hours as every shop has a closing day and it can be very frustrating.
The man from the Braccione shop in the same street was very forceful, but his products are excellent. You can buy the Costiglione de Mulo ( the mule's balls) anywhere though. One thing we looked for was if stuff was kept out of the refrigerator so that it would keep for the duration of our journey. We were totally fine bringing it back to Europe from Italy, but in the US you'd be a bit stuck. Apparently if you put it in your carry on bag as food you are going to eat on your journey you can get away with some of it. Warning the truffle sausages and the wild boar sausages are very strong as is the truffle cheese. But again that can be purchased vacumn packed and that's a great help.