Those are the round cross section o-rings. I am pretty sure by "double seal" jamil meant the square cross section ones like these:
http://www.mcmaster.com/#o-rings-cord-s ... es/=6tdfllHowever he said they should be silicone too. I think he was mistaken as I can not find a supplier for the square cross section (double seal) and silicone, you can have one or the other it seems.
I am about to replace all of mine (and probably most of the poppets too) and I think I am going with buna and square cross section for posts and dip tubes and silicone for the lid.
Either way, once you know the size which is in the post above you can easily find whatever cross section type and material you want on mcmaster.