I was missing one of those when I recieved my Mini Davide. But what I did was contact the site I purchased it from and they sent me one. You might want to try doing the same, or even try to contact AnyVape.
Or you could find out the size of the Davide and ask whoever your looking into buying the ProTank2 o-ring from if the ProTanl's center tube is the same diameter. If they are the same size it should work.