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.