You don't need to find it... you choose it. Go to remix and choose the bind button and the show cards button. Then, you need to open the cards menu (in-game) and finally you need to choose the card, hold the bind button you chose and hold the one you want to choose for it (for example, if you want "B" for the vulture card, you have to hold the bind button you choosed in remix and "B").

Same for me, when I use the ability button the card appears on the corner of the screen but nothing else happens. Tried to use analog, directional input, wasd, arrow keys, nothing works, I've put 6 initial cards on each to try them out, I'm aware that I need more than 1 card.