To find a specific Pokemon card, you would have to find which set the card is in (you can discover this by looking for any of these symbols in the bottom right corner of the card). Then you must buy a pack from that set and hope it contains the card you want. There's really not much of a way to cheat here... just close your eyes, grab a random pack and hope for success! Or, of course, you can just buy a specific card online and save yourself the struggle.
Hope I helped. :)