For sure there are some starters or other things in your local health store. Do you have Whole Foods for example? They sell some dry sruff, not sure if it is a starter or bacteria.
To trry to make from existing kefir. Warm up milk, it should be very warm, but not hot. It will kill the bacteria, add kefir, mix well, leave it overnight on the counter, you can even wrap it in a blanket, by the morning you should have something usable. The approximate proportion should be about 1 galon of milk to about 16 onces of kefir. This is very rough. You might have to tweek with it. Or maybe somebody has better recipe.