That's true with RNG-found items, but brass is literally everywhere in the game world. I'm actually annoyed that they added a chance to mine brass with points in the strength tree. Not only is there a ton of brass already, but mining brass is not a thing. Brass isn't mined, it's created by combining different ores.
To the OP, as others have said, make sure you get the proper books that allow you to find more brass in loot and in harvesting doors. Loot trash piles as there is a good chance to find brass there as well. And despite this game being set in Arizona, there are a stupid amount of radiators inside POIs. Wrench them, smelt them.