I agree with the notion about the bottled fountain of youth, there should still be a price to pay. You could make the characters physically addicted to the fountain water, so someone who would use it would be more irrational, be prone to use more magic, since he knows he can replenish it as much he wants, etc. Maybe even have them "boil over" with magic, if they drink too much water without releasing it as magic, leading to all kinds of side effects.
The vampire idea was brought up as well, and it is yet another dark way to replenish ones magic/life.
On a similar notion, a carrion eater. If people in your world have a fixed amount of life force, then some will probably die before it is used up, maybe they are murdered or die in an accident. Magic users might be able to suck the remaining life force out of those bodies and add them to their own. Would probably be more morally valid (that is, if they don't actually eat part of the body), but also not as powerful. Especially interesting, if some of the dead persons life will actually be mixed into the magic user and change his personality slightly.
Another idea, though I guess it should be exclusive to one character, would be to not use ones life to fuel a spell, but quite the opposite, you use your own death to fuel spells, not shortening your life but pushing back the time when death will find you. Which would mean, that the character would live longer and longer, the more he used magic. One problem with this would be, that it is probably going to be a everlasting life without everlasting youth thing, though you could remove that, if you say he is not only fuelling his spells with his death, but also decay and stays young that way.