If I have Undercity Informer out and more than one mana, can I use his ability repeatedly?
{Undercity Informer}
Yes you can, as long as you have the mana and the creature to sacrifice to activate the ability.
I don't know the exact ruling but compare it to {Ætherling}. You are allowed to spend 4 mana to pump the ætherling up to an 8/1 using its third ability. Meaning the ability can be activated 4 times assuming you meet the requirements to activate the ability.
If {Undercity Informer} was required to be tapped then you could only activate it once because tapping the creature is apart of the abilities activation cost.
RESOLVED
Note that you will have sacrifice a different creature each time you activate the ability. It's part of the cost - you can't stack sacrifices as they don't use the stack, and you can't sacrifice a creature you've already sacrificed. But, if you have 10 creatures to sac and 10 mana open, sure, there's nothing stopping you from activating the ability 10 times!