This weapon will be unlocked after completing the Trevor Philps Industries story mission. It will not have to be purchased.
You can find this weapon for free, use the 'Letter Scraps' video on the collectibles page, and in the cave with letter #30, dive underwater behind the boat and it will be under there by a rock.
Or, you can just get on top of the bridge just one street north from the impound. It will be sitting on the north west arch. You'll need a motorcycle to get on top.
I have found in weapons.meta file that there was also a smoke grenade launcher .So I decided to make a mod for that.I know that it is not a big mod or was hard to do but some will appreciate it.
Big thanks to @GeorgeZhang for helping me out.
This mod will replace the grenade launchers ammo from explosive to smokebomb.