You can buy the start on ebay. I grow it and Kombucha. The start of the Kefir looks like a really big clump of cottage cheese. I can put it in a pint or quart of milk straight from the fridge, and set it in a warm place. Tomorrow I'll have what tastes sorta like Buttermilk. I'll then strain it and keep the big piece that goes into another pint of milk, put the rest of it in the fridge because I like it cold. By lunch it'll be nice and cold for my taste.
It's teeming with probiotics and tastes great.
I grow Kombucha I brew from what is called a Scoby, looks like a waxy pancake, put it into sweetened tea, a splash of vinegar and let it ferment for a few days.
Check it out on the Internet. This is also teeming with Probiotics, and it tastes good. Records go back hundreds and possibly thousands of years, so it isn't a new invention.