Ammo in The Outer Worlds is useful throughout the entire game (unless you fight only with melee weapons). You can find a lot of ammunition while exploring the world - by looting dead enemies or opening various stashes. However, this may not be enough - because of that you should learn a way to acquire more ammunition, regardless of the type of weapon you are using.
How to get ammo in The Outer Worlds?
A universal way of getting more ammo is to unlock a perk called A Few Bits More. It increases the amount of ammunition available in shops by 100%. Get this perk at the beginning of the game. Then, buy ammunition whenever you talk to a trader. Thanks to this perk, you will quickly get a lot of ammunition.