You can make Kefir very easily. Is similar to yogurt but tastes better because it has also some additional yeast fermentation to the lactic acid.You do not need any device, just a lockable container and the bulb.
Where do you get cultures (or determine which are naturally present)?
just buy yogurt with live cultures. back when I used to make my own yogurt I would just put a tablespoon of dannon into a gallon of whole milk.
If you look at the box, it'll say active culture present. Any yoghurt with active culture will do. Once you have your first batch ready, you can reuse it.