I have a lot of cards. I couldn't tell you where I 'won' them. Also, keep in mind they will be given out as Participation Awards, so 'earning' one often times simply means showing up at the event. Lots of store will have extras, and use them as prizes at small league nights and such. So...yeah, they're not particularly Hard to Get.
Now, the challenge coins that only the winner gets? Those are a bit more special. Considering my odds of never making Top 4, let alone win it all....yeah, I'll be begging friends and/or eBaying me one of those!