Doesn't matter who has 'stocks' of it, quite a few countries have the means to produce it.
I'd guess the amount used in the attack doesn't come anywhere near needing a 'stockpile' of the stuff either.
I'm not saying Russia didn't do it, and I'm not saying they did. Until I hear of clear and irrefutable proof I'll keep an open mind thanks
There's a reason we no longer have the death penalty here, something to do with circumstantial evidence and motive proving an innocent as guilty . . with no way back.