If your cue is missing the tip and you need a quick and easy fix, then these plastic slip-on ferrule and tip combo pieces, made by Action, are perfect for solving your predicament. You can purchase these replacement tips to slip over a 12mm or 13mm tip.
This is an inexpensive and easy way to fix a tip that pops off but is more suitable for one piece bar/house cues. They are also a good thing to have in your case should you need a temporary replacement to get you through a match if something happened to the normal tip. The sizes available specify the inside of the replacement ferrule: 12mm or 13mm tip