Probiotics and how to get them

There has been a lot of writing and research on probiotics lately. But how does one actually get a hold of live cultures, and how do they get past the stomach acids?

Will drinking regular store bought Kefir actually do anything?

What actual proven sources for the most useful strains are there?

What should one look for in a probiotic supplement to know if it is useful? Some people will claim anything not refrigerated is not worth your time. However essentially no probiotic supplement found online will be refrigerated. Does dry frozen work?