I keep a sled filled with water in the shade. It is funny how many of my girls will stand in it to cool off. At first, none would use it. I put a few birds in, held on to them so their feet would still be in the water, then let them stand on their own. Of course they took off, then shortly after came back on their own and tentatively stepped in. All it took was a few birds to learn it, and the other see it, and then BAM! They all do it. LOL.
ALSO, I will hang frozen gallon jugs of water in the coops at night close by the roosts, but not so close they can't get up. I put them in the meaties coops also. With the fans blowing past those hang ice blocks, it helps cool a bit. I have also removed nearly all the shavings from my main coop, and will hose off the inside to cool it down. Of course, this also means that I have to scrape poop every morning, but, what ever. It makes it a lot nicer for the girls to have their coop seriously cooled off before they get closed in for the night. Maybe I am risking their health doing this, but I don't think so much...