The M32-Hammer is the grenade launcher weapon that appears occasionally. If you complete the game on a high difficulty, you can automatically get it into your hands through the bonus/rewards menu (and on a high enough level, infinite ammo!).
If you haven't done this, one place you can readily get hold of one and farm kills is in chapter 19 (The Siege). The opening part of the chapter has you fighting you way down hill, then past a mounted machine gun, then up through an alley. When you come to the dead end, Tenzin will open a door into a nearby building. You'll climb a few ladders (killing a bad guy or two) and reach a new battle on some rooftops. This is the key location as there is a checkpoint right as you reach the roof.
From the starting location on the roof, jump straight across to the roof in front of you. Turn to your right and see two enemies on the roof of the next building. Take them out and jump onto their roof. Walk to the far side and look down and to the right. There is a raised area below you, which is likely empty of anyone, but upon you reaching this spot, it will trigger two more enemies to climb up from below in a moment. One of them is carrying the M32. Kill both, jump down, grab the weapon. You'll have 6 shots with it, so use it to lay waste to those around you, then reload the checkpoint to find yourself back on the original roof and repeat until you reach 50 kills.