To ensure your success with brewing Kombucha it's important to start with a high quality Kombucha SCOBY (Symbiotic Culture of Bacteria and Yeast), the ‘mother' that contains the beneficial micro-organisms that ferment the sweet tea. Because your brewing medium is tea with lots of sugar in it, it's also possible that mold spores will get in from the environment and start turning the sweet tea moldy before the SCOBY gets a chance to lower the pH to the point where the mold can't grow. To prevent this you add some already fermented kombucha tea, often called ‘starter tea' which lowers the pH of your brewing medium and helps to kickstart the fermentation process.
There's one place I trust when it comes to Kombucha Cultures and Supplies, and that's Kombucha Kamp. Their SCOBYs are grown in organic tea and sugar, and they carry everything you could possibly want for brewing your own Kombucha. They have many nice continuous brew systems, heaters for keeping your kombucha happy in the cold winter months, very nice tea blends, pH meters and lots more. Hannah, the owner, is super nice and helpful, and is truly passionate about kombucha and helping people to successfully brew their own.