Those are three pound buckets of Grande fresh mozzarella. You can get everything from the little cherry tomato sized ones up to these large balls, but they all come in 3lb buckets. Sumaleria Italianna sells it and they will ship (from Boston) along with the buffalo mozzarella that Dom Also uses. The good news with buying from them is that they will sell you less than 3lbs at a time. If you call around to your pizzeria supply wholesalers in your area you should find one that carries the Grande line. Many of the places that sell the grande dry loaf cheese also carry this fresh (in water) cheese. Unfortunately you will not find it in any normal grocery stores, but if you are nice, and buy a decent sized order you might convince a wholesaler to sell to you. I often combine my cheese purchases with flour and tomato purchases to get my order size up. You can also get cases of fancy spring water, cases of olive oil (great Christmas gift), and other items that you would get at a Costco or Sam's Club type place.
I have found the Grande Fresh cheese to be my personal favorite cows milk mozzarella. I always hear people talking about the wet packed cheese making their pies too soupy, but this is only a problem for me if the cheese is not fresh. If it is in between fresh and not so fresh you can slice it and lay it out on paper towels to dry. I change towels anywhere from 3 to 10 times depending on how fresh the mozzarella is. You will also need to salt it (and the buffalo mozzarella), but when you do you will be rewarded for all your work.