You do not have to buy a culture... make your first batch and "seed" it with a bottle of fresh, raw (raw is important) unflavored Kombucha from the store and your batch will form its own culture...
I discarded my first batch as it got pretty strong before the culture was strong enough to handle, but after that, I had more culture than I needed. I could easily make 6 to 12 gallons a week, but usually only make 3 as this is what I use.
I got a $10 zapper from http://www.zapperplans.com
I used this for several months... only occaisionally now as I also have a terminator...