I have read that growing your own scoby from a bottle of commercial kombucha is not a good idea. Apparently, home-grown scobies are weak and may lack the proper microbial strains. However, I still wanted to try it! Unfortunately, it did not work.
First, I made a cup of sweet tea (1 tea bag and 1 TBSP of sugar), which I mixed with a bottle of GT's "synergy" kombucha. Then I covered the jar with a coffee filter.
Initially, the scoby grew really well; in 22 days it was about 1/2 inch thick and looked perfect, so I figured it was ready to be transferred (along with two cups of the liquid). I took this picture before I transferred it.
Once the scoby was transferred to a larger jar, it died. After that, new scobies formed and died. Even when the scoby looked OK, the liquid remained sweet, so the cultures were clearly not active. After 2 months, I gave up and bought a starter from a reliable source (and now my kombucha is thriving!).
Other people may have better results than I had, but I would recommend saving yourself the hassle and just buying a starter.