What ever you use please, please make them sealed motors.
The cheap box fan you see hanging in so many barns are not safe...just not safe.
Dust, dirt, cobwebs are highly combustible when near a ignition point = fan being turned on/off.Barn fires are commonly caused by electrical issues = fans that are not sealed motors go poof from the dirt, debris...
Now, they do make a sealed motor box fan that hangs stall front but it is as costly as wall mounted fans that are permanent and no horse is touching up high, knocking it down.
I rented a 8 stall center aisle barn although I only used 4 stalls, each side was wired for 4 fan operation.
It had 18" wall-mounted fans in the barn that were hard-wired for rodent proof safety of my horses just in case.
The fans were secured to the stall wall up high and angled so the horse no matter where they chose to stand was covered in a cooling air-flow in their stall.
Each stall had a dedicated fan.
All the fans were hardwired on on/off switchplate...all wiring enclosed in steel conduit for safety reasons.
There are many places to purchase sealed motor fans from via the internet or actual stores local.
I know Northern Tools, Farm-Tek, Grainger, certain times of the year Tractor Supply, and Home Depot has availability to order in but not sure they store stock 18" & 20" wall mount, sealed motor with a tilting head for directional flow...
The prices are varied you will find via a internet search.
You need to know your needs you are dealing with as some fans must have a minimum above ground for optimum efficiency.
How your barn is constructed and where you plan on installing the fan to hang from is all part of the decision of what you choose.
Ceiling mount, wall mount would be my choices not aisle free-standing as a accident looking to happen is not my idea of fun.
Oscillating or stationary directional are more choices to make.
I also have a 18" oscillating fan in my house garage we use when doing a project in the garage we have air movement..
My garage is 24'x 22' and that fan moves air in all corners so since most stalls are not that large a size, think the need to go larger than 18" or 20" would be unnecessary...
Personal choices to make...
Enjoy the search.