Not that I know of Albert7777....the only thing I know of as a preventative, is the medicated feed, and once sick, the meds. I'm so sorry it's so far away

Just keep doing what you have been doing and know you are doing your very best! Are you sure the feed store doesn't have any meds? Here, we buy a med called Safeguard liquid and it's for goats. Some OP's figured out how much to give to chickens, we call it "off label use" but it works. It's the Amprollium. sp? Not sure I spelled that right and I'm an adult LOL. It's late here. You could always ask your feed store if they do have the meds for "cocci" or something that blocks thiamine. The medicated feed bag label should have the name of if on it. In the future, just know that it's better to feed the medicated for a couple months after they hatch, so you don't lose any more batches of chooks. Wish I could help more
Try a Private Message to the lady that said she would help, as she lives closer than I do, if you don't hear back right away...that's the best way to get in touch with someone on these boards quickly and she sounds like she can help...wishing you the best of luck and glad you have someone who can help you better than me!!!