The polyvisol vitamins can probably just be found at your local grocery store. I've never had to purchase them myself, so I'm not 100% sure, but you can definitely try there. As for probiotics and electrolytes, you should be able to find these at your feed store. Save-a-Chick is a common type of...
They might have a vitamin deficiency. Get some infant polyvisol vitamins (without iron), and start giving each of them 1-2 drops each day. See if this helps. Vitamin B and E deficiencies can cause inability to walk/stand, as well as some paralysis.
Continue making sure that they are getting...