I think Raw Kombucha from the store is okay, but after buying a few bottles I started making it myself and have been for a little over 8 months now.
A little Sencha or Bancha green tea (I prefer the green tea), cup of raw organic Sugar and a full bottle (two servings) of Raw Kombucha from the store and I had my first culture. The first batch took a long time because I was growing the mushroom from nothing but the raw kombucha in the store.
I make three gallons every 15 days or so. About every fifth day I brew a new batch using the mushroom and about eight ounces of brew from one of the older batches.