You can find kefir in health/organic shops like Planet Organic, Whole Foods, etc. that have nothing else added. I agree, don't buy anything with any form of syrup, sugar, since it obviously defeats the whole point of the diet.
You could try and make your own from the start anyway, buying the grains on ebay, if you're worried about the price. Cultures from around GBP4. I'm starting on this, just got them yesterday and the buggers effing stink, I hope I'm doing it right.