This bottle popper is designed to allow up to 4 tritium inserts in it. It also is perfectly fine using without the Tritium Vials also. If you want it on a keychain or paracord, just leave a vial out of the section you'd like to run it through.
The size needed for the tritium inserts is 3mm x 23mm. The holes in the design are 3.25mm due to inaccuracies in the printing process. The tritium vials will need to be secured with glue, and the end user will likely need to drill the holes out to make sure you don't break the vials.
In order to find tritium vials, I suggest http://www.candlepowerforums.com/vb/forumdisplay.php?189-CPF-Kiosks and finding someone to source them. Longer vials will work on the top and middle, but not the two at the bottom