I've never seen commercial kombucha. I'm not sure if it's available down here (I'm in Australia), so it's not really an option for me. I can easily enough get a kombucha culture from someone else if needed, but I get a bit sentimental about keeping my own colonies or cultures of things going.
I think people are a bit fanatical about kombucha and tend to think it's a bit more mystical than it is. It's basically just yeast and Acetobacter, so yeah, you could just stick some raw vinegar into some sweet tea and voila, you're making kombucha (it might take two or three cycles to start working like normal kombucha).
I keep at least half a dozen cultures going all the time so there's always something I can use. My cultures do end up working if I use old stuff to start them, I was more just curious about learning anything other people knew.
Thanks for the responses