Ray Gun

devil5care

Soon after the quest for sending grem-aliens to another town, Cletus will get an ! over his head, and point out that Kodos is flying in his UFO overhead. Frink will ask the player to buy the Ray Gun to shoot Kodos down.

Reichljuice

I had the same issue, but it's exactly like the previous commenter said. Cletus notices it before it becomes available for purchasing.