This trick requires two memory cards. After obtaining the desired cards, save the game on both memory card slot one and two. Then, start a game in multi-player mode with saved games from both of those cards. Bet the desired cards from memory card slot two, and intentionally allow the player using memory card one to win quickly. When you have obtained all the desired cards from memory card slot two, save the game from memory card slot one again into both memory card slot one and two and repeat.
This trick requires a little bit of time, but is well worth it. On your file, get the card(s) you want copied. While in story mode, save your data over the second file and yours. You now should have two files exactly alike. Next, create a third file from scratch. Once you are able to save that file, do so and exit back to the title screen. Go to versus mode.
Note: This requires two controllers. With the first controller, load the second file. With the second controller, load the third file. With the second file, bet the cards you want copied. With the third file, bet any card. Battle and allow the third file to win. The cards you want copied are now on the third file. Next, battle again, but this time, have your file against the third file. On the third file, bet the cards that you won from the second file; and on the your file, bet any card. Battle, and allow your file to win. The cards that you wanted duplicated are now copied. Go back into story mode and resave over the second file. Repeat this as many times as desired.