Quite right, Rubah. Visit Cheryl's shop repeatedly, and (eventually) you will get a Rosetta Stone, and the other one I was referring to is in Ultimecia's Castle. So that question is done.

I've never heard of getting one from the card guys. I guess it's just really rare. I get all the ones I need by refining Curse Spikes into Dark Matter, into Shaman Stones, into Rosetta Stones, it takes a while, but you don't need that many of them. 6 at most (and even that assumes you didn't get Bahamut).