Runes are found randomly in MyTEAM cards. There is no one player/card that guarantees a rune will appear.
You must got to collection and select a player card. Pick the view card option and use
to rotate the card. Rotate the card to each of the four corners. You will know if you have found a rune if the left tab that indicates the player's color has a "glitched" look, followed by a rune symbol.
EDIT: Markyshizzle made a video explaining runes and the black market. I suggest you watch it as he gives a more in depth explanation.