You'll find enough of grenade launchers in the campaign to get 20 kills. You can even kill multiple enemies with one shot if you're good (or lucky, like me). You must get these kills in the Perseus Mandate campaign.
A good way to horde more ammo for this gun (AND EVERY OTHER GUN) you already have in your inventory is to swap the K3-BT for another, then swap thát gun for the second K3-BT you just killed for, then pick up the first K3-BT you dropped. This will give you MORE ammo than if you were to pick up a second K3-BT when you already have one in your inventory.
This is because the game gives you a bunch of ammo if you pick up a different gun, but less ammo if you already have the weapon in your inventory.
Example:
if you swap your pistol for the Penetrator, you'll get 75 rounds. If you pick up a second one you'll get only 10 more, giving you a total of 85.
But if you had swapped back for the pistol and THEN picked up the second Penetrator, you would have had 2x 75 rounds, totalling 150 rounds!
NOTE: these must be weapons DROPPED by enemies, not the ones that are lying around for you!