No, I lost you for a minute at first. Sauerkraut is high in probiotics as it's fermented. I spoke too soon and corrected what I meant when you asked how much.
If chickens actually eat this stuff on a regular basis, then I'd guess a tablespoon or so per day is plenty to give them the...
I would assume like my fermented feed..., but a little less as it won't have the nutrients mine has. I give them 1/4 cup each every other day. For sauerkraut, I'd guess a tablespoon per day for the probiotics, but to fix the screw, I suppose way more than that!
You can find fresh sealed in plastic. Our grocery store has it over by the pork chops. There's also canned in a can, and canned in a jar in our store as well.
We usually just get this as it's the smallest portion of all three choices.