I acquired my grains from GEM cultures. It was a little spendy, but it is a one time expense and GEM has a long established reputation for quality cultures.
You will soon have more grains than you can possbily use, kind of like microbial zuccini. Give some of those grains to your friends and other intereted locals for free with the understanding that if your grains ever die, they will return the favor. Kefir culturing has great rewards for very little effort. You're not likely to develop carpal tunnel syndrome from pouring a little milk into the kefir jar once a day.