I never came across an electrolyte mix for chickens. In our extreme heat, I kept huge ice cubes and huge bowls of frozen water with herbs mixed in, in the freezers. In the afternoon I would take one of the huge bowls and 3 - 4 of the ice cubes out and set up their lap pool. I'd put the ice cubes in that and leave the huge bowl on the ground for them to pick at. These were both inside the coop, out of the sun.
I cleaned out the freezers yesterday and discarded all the frozen things from last year, as I don't have chickens this year.
That's an awesome tip for helping chickens in a heat wave! I like it! Thanks!