*This solution was created by SaintedKarma in the XB1 version of this game*
Barnstorming locations are the only collectible that aren’t displayed by the collectible finder, thankfully though there are a total of around 140 in the game so you only need to find a little over half of them for the Achievement and challenge.
After i had collected all the Soul Clusters and completed all the activities/ diversions/ missions i had around 55-60 out of the 75 i needed.
Here are some easy places to try for the rest if you are in a similar situation as i was, it helps to have the flight power fully upgraded but not necessary.
-1- Try the arches dotted through and around the wall in downtown odds are you haven't flew through all of them.
-2- Try under all bridges.
--- bridges above lava
--- the motorway overpass
--- bridges in between buildings
-3- Try all the buildings in The Slums with holes through them.
(I'm not sure if each building has 2 barnstorms or just one. For example if the East to West hole & North to South hole count as the same one or two separate barnstorms)
-4- Try all pipes hanging from cranes in The Forge.
-5- Try between the statues legs in The Forge or any low arches.
-6- If you've done all the challenges/activities and tried all of the above and it still hasn't popped try flying between close buildings you might find a few.
Barnstorming Definition : (well my understanding anyway)
A Barnstorming location is basically any location that has two walls on either side and a roof over top, so that you are flying through the opening in between. You should be on the look out for locations such as this while doing collectible cleanup, as a large number of soul clusters are in these such locations, so make sure to always fly through these areas to pick up the location.