The Ratslayer can be found in the Broc Flower Caves located at the southeastern region, between Nipton and the Bloodborne Cave. To locate the cave without aid of the Explorer Perk, findthe Atomic Wrangler owner and talk to the sister Gerret. Accept the work of Debt Collector, then find a woman and persuade her to cough up the money. She'll give you half of what she owes and gives you the exact location of the cave. In the cave, follow the tunnel until you find the cliff edge of the main chambers filled with massive rats. On the edges of the main chamber, follow it to a room where the Ratslayer is sitting on the desk to your left. Inside the rest of the room are lots of medical supplies (stimpaks, doctor bags, etc).