You can buy pre-made kefir at most health food stores. Some, depending on the quality, may contain higher levels of natural sugars than others.
If you want to make it at home, the best choice of milk is unpasturized goat’s milk. Unpasturized milks contain more enzymes and probiotics than pasturized milk. However, it may be difficult to find unpasturized milk depending on where you live.
We allow kefir and greek yogurt because they contain probiotics, which is the single most important thing you need for your recovery. But if you take probiotics without doing the strict diet, they will simply just die-off and not remain in the body very long.