Once you have used all of the easier outfit unlocks, (deficit comebacks, win with forehand, backhand, etc.), the best way, and possibly the only way to unlock the rest of the outfits, is to win 22+ matches with each character.
**Only the matches that would unlock you outfits (22nd match, 23 match, or higher) need to be played against a computer. The rest of the matches can be boosted with two controllers by yourself.**
To clarify, matches 1-21 (all wins) can be played in exhibition with two controllers. You will only need to win matches 22, 23, or higher against a computer (easy difficulty works well).
*To change match settings: from the main menu, go to options, then game options.*
-games per match 1 of 1
-points per game 7
-points per serve 5 (you may prefer 2)
*To check how many wins you have with a particular character, from the main menu, go to extras, and then stats.*