If it's live rock from tubs at your LFS
, I doubt you will need to add an ammonia source. When you add the rock to your aquarium, there will most likely be die-off from the rocks, and those things that die-off will provide all the ammonia necessary to begin your cycle.
Put the live rock in your tank, hold off for a day or so, test the water chemistry and if you don't see any ammonia, wait a couple more days to test again. If you still don't see any ammonia, then I would say you had minimal die-off and the nitrifying bacteria on the live rock is doing what it's supposed to and you should be good to start adding livestock.
Still, don't rush anything. Give the tank a few days as you will most likely see die-off from the rocks as that is pretty much how it goes unless you bought some seriously cured live rock that has been sitting in buckets for a long long time.