Some people grow kefir in water. I do not, I use milk. I use whole milk because that is what I put in my tea, but you can use whatever is cheapest. Even mix up powdered milk. It's good for you, and good for the chooks.
For probiotics, have you tried kefir? Once you get it started, you don't have to buy it you just keep adding milk and it takes care of itself. The kefir will separate the milk, even if it is homogenized. The whey has probiotics, so you can skim that off for them. I don't know if they will eat...