Sources of bacteria are readily available especially for us Aquariests...
I ask, where did you buy your tank, filter, gravel, fish food, fish, plants - shall I go on?
The point is, although the bacterias can be 'borne' via the air, I personally feel that we get more than we need from the LFS we visit as the bacteria reside on everything in the store (even us when we leave).
...ammonia is not sterile and yes, it contains ammonia-bacteriums, but no, bacteria cannot "come from ammonia", rather it feeds on it. *
tips hat to paperdragon*
Do you know how expensive sterile ammonia is!?!?

It has to be "built" in the lab! Or at least that's what we had to do if we needed a pure source.