Greetings, welcome to the Forums.
I don't know of a commercial source offhand, but if you know of Palo Brea trees planted in your vicinity, consider collecting seed from them. Though it is less commonly planted than some other Palo Verde, there are doubtlessly some street trees and plantings in nearby botanical gardens. Of course, seed collection may be frowned on at some sites, but it is likely that other places would consider pods and seeds to be debris of no concern. All of the various Palo Verde (Parkinsonia spp) have differences involving bark color, leaf size & shape, height, and other traits, but overall they are quite similar. I am curious why you seek Palo Brea in particular.
Palo Brea (Parkinsonia praecox, formerly Cercidium praecox) is one of several species of Palo Verde (Parkinsonia) grown as ornamental trees in arid and semi-arid landscapes. Parkinsonia praecox occurs natively in dry savannah habitat from Mexico to Argentina.