On another note, what is the difference between sziadle mleko and maslanka?
Real zsiadle mleko is fresh raw unpasturized milk that has been allowed to sit out at room temperature overnight until it gels from natural bacterial action. The cream floats to the top and hardens into a chewy layer that sits on top of the gelled milk. The stuff you get in the store is pasteurized milk to which a bacterial culture is added, and perhaps also rennet. It resembles rennet custard in texture. The texture is uniform without the cream layer on top.
Real maślanka is what is left in the churn after you make (sour) butter. The milk used is unhomogenized and left to ferment slightly so that the cream can separate out and form the butter. After you scoop the butter out, you're left with maślanka. The stuff you get in the store is made with a bacterial culture.
Good luck finding live kefer grains. Don't get the dehydrated ones, and make sure they are for milk kefir, and not something called "water kefir".
Real kefir is fun to make and a thousand times better than storebought. It's nice and refreshingly tart, and fizzy with a little alcohol in it, but not enough to get you drunk. It has a pleasant yeasty flavor that the storebought does not have. The kefir grains are a symbiotic clump of various bacteria and yeasts. It looks like a piece of cauliflower, and it grows every time you use it, so you can share it with your friends when your piece becomes unmanageably large.
Also, if you're ever back in Poland, the best commercial dairy products are made by Piątnica. MUCH better than other brands like the ones you mentioned.