There are a number of pocket knives found in each map. Just keep searching the cabins and get Jason to grab you. Your counselor will automatically stab him. With all these crashes it still counts towards your achievement but not the badge if you lose connection. So if you find multiple knives they will each count toward the 13. Good Luck.
MiggyBombs added The latest patch Tommy Jarvis spawns with a pocket knife. Not guaranteed that you spawn as Jarvis but it adds another way as opposed to finding knives.