First, acquire Stage Builder Parts Set B (Make 5 custom stages). Make a level where there are conveyor belts running into a wall, and a platform somewhere in the level that a player can stand on and stay there.
Plug in a second controller. Go into options, and set every item EXCEPT Sandbag off. Set items to high. Go into your conveyor-belt-level in a regular Brawl and have the second character jump and stay on the platform, so it's not in the way.
Now, when the Sandbags fall, they'll stay at the wall. Use your character (Kirby's basic A attack, repeatedly, recommended or another character that can hit many times quickly.) to hit the Sandbags over and over. Since all other items are turned off, only stickers and CDs will appear. This is the easiest way to get a lot of CDs/Stickers quickly.