Cokin themselves don't produce 10 stop filters, it's Lee and Hitech who make P series/Lee sized 10 stop filters and Hoya produce a 10 stop screw in filter. Its unlikely that you'll find a 10 stop in A size as A size is TBH not as popular as P size because it only supports a max of 62mm lens filter ring size and a 10 stop is still a pretty professional piece of kit. Buy a P Sized filter holder/rings and then get a Cokin A-P adapter so you can use your A size filters and then get a P sized 10 stop. Easy.
BTW, I wouldn't bother with Cokin ND filters, their nortorious for producing a magenta colour cast.