I have been making yogurt for years. I used to refresh the starter every few batches with some freeze-dried yogurt starter or a new cup of plain yogurt from the store.
Then I read in Sandor Katz' book that these are commercially controlled starters, and it's possible to acquire wilder starters that are self-regenerating if properly fed.
Consequently, I bought a Bulgarian yogurt starter online (here) this summer. As long as I remember to save a little yogurt and make a fresh batch every week or so, I will never need to "refresh" my starter by buying something again. I love this independence (which comes with the caution to keep the Bulgarian starter alive!).