From what I understand tritium is the easiest but there are other ways to acheive fusion. Once we figure out deuterium-tritium fusion we should be able to work towards something like deuterium-deuterium fusion. You also have methods that can be used to manufacture tritium. One of the later stages of the ITER project is to attempt to use the reactor to breed tritium. If they can pull that off then there would be no need for external sources of tritium.