If its a Taco 007, then yes local box stores carry it. Depending on a number of factors 007 should be able to handle all 3 zones. Sometimes you can just swap the cartridge in those, however savings vs full replacement are minimal. Circulator failure is highly possible, however you might also be airbound badly. Can you shut off individual zones and fast purge them one by one? I have certainly had occasions where zones just fail to heat due to air, especially if the system was opened for some sort of service or repair.