Texan O Rings

Anyone know what size O rings the .45 cal Texan uses? Specifically I need the size for the o ring on the air bottle that is in the first groove. It is covered when firing and visible when loading. Very thin but about a half inch in diameter. Of course a list of all sizes would be helpful.
I'm pretty sure Air Force chose standard size O rings and not something proprietary. It would be nice to be able to buy them locally.