I have Ramm basket fans in each stall and a Ramm ceiling fan in the aisle of the barn. Those fans move a ton of air and are closed motor fans, so safer than standard box fans.
We also recently renovated the barn and opened up the gable ends with slats (like lattice) for increased air flow. Previously there had been some sort of solid fiberglass panel. At the same time we widened the aisle doors, which also increased air flow.
We are also getting ready to have insulation added under the roof. It will be sprayed on. Apparently the down side to that is that it could hide future leaks, but we have decided that cooling the barn is more of a priority.