I make it myself all the time! Pick up some organic ginger (so it still has microbes on it) and some distilled or spring water (or leave tap water out overnight to dechlorinate). Start with 1 cup of water and grate in small piece of ginger (skin and all) and add 2 tsp sugar. Repeat once a day for 3-5 days until foamy.

Then take two quarts of water in a large pot, add 1.5 c sugar, the juice of two lemons (this is important! It doesn't work without them), and as much ginger as you want, bring to a boil, and then take off the heat and add 2 more quarts to bring it up to a gallon. Allow to cool to at least 90 degrees F, then mix in your starter and strain into bottles (I like to use plastic seltzer water bottles and strain through a fine mesh wine filter bag). Leave it in a relatively warm place for a week or so, burping the bottles occasionally as the pressure builds. Sometimes not all my bottles will take off, so I'll pour a little from one of the successful ones in to restart the fermentation.