Go back to Trabia Garden to get Selphie's card. I believe you have to play her friend, a girl sitting at the fountain. If its not one her, then its someone in that general area.

And yes, you have to get Zell's card from his mom in Balamb Town. There are certain times I've went back to try to play her, and couldn't though, so its only at certain times in the game at which you can challenge her. So if there is an instance in which you can't challenge her, just come back later. I challenged her, and got Zell's card at just about the end of the game, right before finishing it.