Cards are 1 in a 150 (estimate), meaning they are pretty rare. So in about every 150 enemies you defeat you may get one card. This is just an estimate that I believe is around accurate.
Also, another thing. As you become higher in Notoriety you will get higher playing cards. For example, I just got my first Ace yesterday at Notoriety level 37, and got another one today at Notoriety level 38. At the beggining of the game, you usually recieve 2's and 3's.
Good luck finding 'em.
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Shhlatkinz 
The Guardians
(remake of The Brotherhood)
Notoriety 40
Pirate Master
Toontown
Cool Corny Electrofoot, Nacho Average Pig, Cool Corny Electrofoot Blak