Most hamburger is made of Chuck (either roast or steak) cut into cubes and ground up. At the store they can add other cuts of meat to this base and cause variations in the quality of the hamburger. But that works both ways, obviously if you add tenderloin you are going to get a better batch of hamburger. But to keep things consistant just use chuck. Chuck comes from the back top of the cow and is fairly tender although it is not one of the prime cuts.
If you have a butcher that has chuck steaks for sale buy a couple of pounds and ask him to grind it up for you. They can do it easily if it is a real butcher and not a supermarket that has not equipment.
