I'm happy to announce that I have kombucha cultures available that I synthesized from a certified kosher source, namely GT Dave's Organic Raw Kombucha, which is certified kosher-pareve by Rabbi Eli Frankel at the Kosher Certification Service of Los Angeles (see Kosherquest for more information). I'm happy to mail them out for free, as long you pay shipping costs. And please only contact me if you're a person who keeps kosher and wants to secure a culture from a certified kosher source.
Despite the fantastic rise in popularity of kombucha in recent years, I have seen nary a peep about it in kosher and Jewish forums, neither online nor in print, and believe me, I searched. I even sent a request to the people at Kosher Blog quite a while back asking them to address kombucha, in particular because it is becoming quite common for people to brew kombucha at home (and it's so expensive to buy), and as long as Kosher Blog is covering pickling, canning, home stock-making, home salami-making, and home cheese-making, I thought home kombucha-making would be quite relevant. Obviously I was a little disappointed that they didn't cover it.
So, I did my own research. I spoke with three different halachic authorities about kombucha, two of them being representatives of their respective kosher certifying agencies, namely the KSA and the KCS (previously mentioned). I understood from my conversations with these three authorities that a kombucha culture is kosher and pareve by nature. Halacha's conception of it is more similar to beer or bread yeast, than to, for instance, wine yeast or bacterial cultures for cheese (yeast (other than wine yeast) doesn't need kosher certification according to Rabbi Eidlitz at KosherQuest, see here, although the Star-K takes a more machmir stance and requires kosher supervision; although, based on the fact that all major kosher certifying agencies don't require kosher supervision for domestically brewed beers, and all beers use yeast, the logical conclusion is that yeast doesn't require kosher supervision either).
A word on the kombucha making process: Kombucha brewing is an artisanal process. The utensils and ingredients must be very clean and pure or else the scoby will be likely to get moldy. The culture is never exposed to hot temperatures, as that would kill it as well. When a batch of kombucha is starting out, the scoby, a saucer-like symbiosis of bacteria and yeast is placed in a room-temperature mixture of water, tea, and sugar (none of which require kosher certification on their own), and is left to ferment for about a week. That being said, because of the dearth of information about the kosher status of kombucha in general, I decided to provide a secure source for people seeking kosher peace of mind.
I personally started brewing kombucha over a year ago. These days, the only liquids I drink on a regular basis are water and kombucha. Why? Besides for the fact that I feel kombucha generally boosts my immune system and gives me energy (I have never been sick as long as I have consumed kombucha regularly, keineina harah) it also coincided with the sudden retreat of a virus I was experiencing prior to when I started brewing kombucha. It was nothing serious, but the virus completely subsided within two months of drinking kombucha on a daily basis.
If anyone has anything at all to add here, or if anyone feels that I was misinformed about anything, please don't hesitate to let me know. And if you want a kombucha culture that was synthesized from a certified kosher source, I'm here for you.