"To one as young as you, I'm sure it [being ready to die] seems incredible, but to Nicolas and Perenelle, it really is like going to bed after a very, very long day. After all, to the wel-organized mind, death is but the next great adventure."
He goes on to to say that the stone isn't really a good thing, to have all the money and life you want.
"The two things most human beings would choose above all - the trouble is, humans do have a knack of choosing precisly those things that are worst for them."
I think, if Dumbledore really did have a spare stone lying around, it would contradict a lot of what he says in this passage, along with what he told Voldemort in OotP about there being things worse than death.
I do think there's something going on that's keeping him alive (150 is just not a normal life span!), but I'm starting to wonder if wizards, on the whole, have a longer life span. I mean, all the "medical cures" we have aren't as efficient as their magical ones. Instead of taking 2 months to heal broken bones, it takes 15 minutes. Hagrid, to our way of thinking, might be in his early 40's, but in the HP books, he's somewhere around 65. McGonagall gets around pretty good for someone at least 70.
But, no, I really don't feel like Dumbledore has another stone hidden away in case of emergency. And I really think people should let Sirius die a respectable death and stop trying to think up random ways he could come back to life.
