I think you are better concentrating on natural prebiotics, by eating lots of fruit and vegetables. Apples in particular are meant to have very good prebiotic properties.

I like General Biotics[1]. It has 115 strains which is an order of magnitude more than any other supplement. One of the founders is a researcher in the field published in all the big name journals. They don't make claims that can't be scientifically supported, unlike most health supplement companies. Highly recommended.

1: http://www.generalbiotics.com/

I had good experience with the Trader Joe's Milk Kefir.

Sauerkraut is a good option if you like actual food.

I've used this in the past:

http://www.amazon.com/dp/B00SXBWDOY

Seems to work. My wife uses it regularly. Just a couple of data points for what it's worth.

I would first do some research on probiotics. From what I've seen, the only proven benefit came using them ina very certain situation which I forget completely (it was something like it helped clear up diarrhea after an antibiotic quicker or something like that). My point is they aren't cheap and you could simply be wasting money. I'm honestly not informed enough either way, but when I looked into them a few years ago, there was little to no proven benefits of using them.