All 3 of my kids drink at least one serving of kefir per day - it's an excellent source of protein, calcium and probiotics. We buy Helios brand at Whole Foods. Strawberry, peach and raspberry are all yummy!
I bet you can use any milk. Goat is used to make yogurt, and since Kefir isn't much different (although it's cultured with grains) the milk should sour up just as well. Just make sure that if you decide to use pasteurized milk, get unhomogenized (although raw is best).