In my home, we make milk kefir (husband) and also coconut kefir (me). I bought my original kefir grains from Happy Kombucha
http://happykombucha.co.uk/collections/all-kefir-grains-and-kits because this site had been recommended as selling high quality grains. They also have useful video instructions, which is helpful. Since getting my original grains last year, I have frozen them, taken some on holiday and given loads away to friends because they multiply really well. I also tried water kefir but this was not good due to significant sugar residue in the finished brew.
Incidentally, if anyone wants to make coconut kefir, check the ingredients on the cans of coconut milk - most of the ones you find in supermarkets contain thickeners and preservatives. Thai coconut milk is coconut and water only. I usually buy mine from ethnic stores but I also found one brand in Sainsbury's.