You can get your hands on the Prismatic Hammer right after you get The Unreliable airborne, which means after finishing the "Stranger in a Strange Land" quest. Start by going to the captain's quarters and boot up the computer there. There is an entry for tracking down science weapons on the computer, so make sure to read those.
Next head to the Groundbreaker and talk with Gladys. You need to purchase a Battered Mardet databad from her for 500 Bit, unless you have discounts to make it a bit cheaper. This will allow you to track the quest on the map, making it a lot easier to find. However, we will of course also explain it down to every detail.
The next step involves you to go to a hidden room on the west side of the Groundbreaker. Go just past the customs checkpoint (coming from your ship), and head into the door on the left just before you go to the door to the Promenade. There is a room there with a lot of bunk beds. At the end of these there will be some boxes that you can jump up on, so do this and run along the path of bunk beds until you can crawl into a small hole in the wall.
When you get past the hole you get to the Repair Hangar. There are many outlaws here, so either kill these or sneak past them. Open the door on the right either by using the Repair Hangar Cardkey (from the outlaw leader), or just lockpick the door. Alternatively you can pickpicket it from the leader. The Prismatic Hammer is found just inside this room, so just loot it, and it is your to keep. Congrats!