finallyfaith
i recommend this site for a wonderful in depth info on kefir.
http://users.chariot.net.au/~dna/kefirpage.html#obtaining-kefir-grains
he does sell his grains, but he is located in australia so it may not work out if you are in the states.
this site sells what looks like good kefir grains, milk based
http://www.happyherbalist.com/index.asp?PageAction=VIEWPROD&ProdID=47
this site sells water based kefir grains. you can make non-dairy kefir with this. you can also make coconut juice kefir. the dairy based and water based kefir grains are separate entities, you cant make dairy kefir with water kefir grains or vice versa. allegedly there is a way to convert one to the other, but i have heard it doesn't work too well, best to just keep them separate.
http://www.fermentedtreasures.com/kefirwc.html
also there is a difference between the culture and the grains. the grains are the mother culture, it will live indefinitely. you can keep making as many batches of kefir with one set of grains as you like.
the culture is only good for so many batches. so you have to buy more culture after X number of batches have been made. i linked sites that sell the grains. there are other sites that sell the cultures.