The zombie sheriff doesn't spawn for me. I have tried getting the armory in 10 different saves but it never spawns. I use the max resource cheat so there might be an issue between the cheats and the game. It had spawned before I knew cheats existed so that leads me to believe that is the issue.
The zombie sheriff appears randomly in 8 different spots around the farm. Look for a zombie with an arrow pointing down at it. If you do a lap around the town you should be able to spot him. Entering and exiting a dungeon will reswpawn and reset the zombie sheriff again.
Hope this helps,