Stay away from store bought milk. They fail miserably. Most are ultra pasteurized and pretty much dead. They are also homogenized which makes really bad texture and the fat behaves totally different. If you can't get raw milk, try to find non-homogenized (also called creamtop, cream-top or creamline) milk from a local farmer's market or your nearest Whole Foods. You want it to be gently pasteurized. Obviously it is far more expensive but it makes the difference between success and failure of this cheese and it's a night and day difference in coagulation abilities and yield. It's also nice to know your milk comes from local grass fed cows of a local family farmer rather than a badly treated factory cattle that munches on corn/ferments, growth hormones and antibiotics (all of which may also fail your cheese).
Remember that when you age cheese, every little nuance in the milk amplifies ten folds. You won't be able to feel these things in your morning cereal or by adding a bit to your coffee, but in aged cheese all of these things come out. You spend so much on cultures, fridge and time spent on this project, it would be disheartening to see if fail because of its #1 component: milk.