@NaughtyAndy
SECs (special event cards) cannot be bought, not ever.
One can get them in most games (free SEC) though this may be rare, I am not sure how rare.
One can get a SEC at the end of a progress bar, which often occurs with the games.
Currently there is the "Summer Sales".
Here you can play games. If enough games are played, you will get the SEC, which is Toree - Sexy Beach Volley.
The progress bar displays how much progress (if any) is made during the games.
Once the bar reaches the right end, the card is yours, though you may need to click something to receive it.
This is also the same with the EX cards which cannot be bough and not available during the games, except at the end of the progess bar. They are rare so they do not appear often. One the event is over, the EX will disappear for everyone who does not own it.
I believe the game does not matter. Any games during the event will work this way.
I have never tried to get a SEC this way, so I am not totally sure.