The 0 level spell disrupt undead deals 1d6 positive energy damage, but only to undead? Doesn´t positive energy hurt evil outsiders too?
And why can´t one use holy water as a power component?
A. Positive energy does not harm any outsiders, unless you have the Alignment Channel of the corresponding alignment to those you want to harm. Unless the outsider also has the undead subtype, in which case positiive energy would also affect them.
B. There is a list of special material components you can use in Adventurer's Armory and while I don't remember holy water being listed, I'm sure you and your GM could come up with something if he's game for the idea.
It's not holy damage.