Right, but brass locations have been mentioned earlier. There is a LOT of brass available in the world. Even when I used to craft ammo, I'd still have stacks of brass sitting around. Try those locations mentioned and you'll find a lot of it. And don't forget the perk/books.
The best thing for ammo is to not use the same ammo as your turrets. If you're using SMG turrets, stick to a .44, 7.62mm, shotgun, or archery weapon, for example. Turrets use a lot of ammo, so if you're using the same kind of ammo in your weapon, you're going to run low. You can also reduce the number of turrets and use other traps instead. Though I do tend to use around 8 turrets on my horde base - 2 in each direction. And sometimes more.
Headshots are the best way to deal with any of the zombies. The two new zombies aren't even really any more difficult to kill, and the blue zombies also die easily. The red zombies are the only ones that need a bit more effort, really. Max out the mods in your weapon for the extra damage and increase your weapon perks and you shouldn't have any trouble killing them without needing a lot of ammo.