you cannot simply 'buy' them.
Some are offered as part of a special event like Advance Purchase of all December cards and Xmas day is the SEC
Others can be won as part of playing the event games.
The game will show which SEC you are playing for.
Sometimes you may win a Joker card which can be traded for any card (not NFT)
for more info, use the Forum Search facility and search for "special event cards"