Yes, you'll find it in the milk section. I particularly like the Helios brand of kefir. I've really gotten into it and I ordered some "kefir grains" from Marilyn in the kefir forum here on curezone. I'm kind of excited about making my own kefir and I'm going to make kefir cheese too. (But then again, I'm kinda excited about everything... :)
As far as how many days. I have just made it a part of my regular diet. It is very healthy and can be eaten everyday. Lots of people take it everyday as part of thier healthy diet.