QM is the only source for the tank shells. You need to start buying them as soon as they appear to get all of them. I think they start appearing in the QM's inventory a mission or two before Newton. Like four or so at a time. I think there are like 12 total. You only actually use the tank in the Newton mission, and after that it will not appear in any mission locale you drive it to. It comes in real handy for random encounters, especially with that gun, but that is all. Just buy them piecemeal and store them in a locker at the bunker, then pull them and bring them along when you get the Newton mission.