Where did you read it was custom? It looks like an actual vintage shirt to me, which would be backed up by his stunt double wearing a different yellow Hawaiian shirt.
It is probably from a designer brand because of his status so... yeah :/ You should go search Bugatchi which is a high pricing exquisite brand of shirts like that. Perhaps they have that.
That's really weird that they custom made it but only made one for Brad and not his stunt double.
If it was purchased or based on a real shirt, it would be from one of the Japanese Hawaiian shirt brands from the 60s. There were a few of them and their shirts are beautiful. They sell for a few hundred dollars now usually.