A way to get unlimited orbs is to defeat all objectives in any level,
but don't finish the level.  Save your game to your memory card
(before finishing the level), go to the option menu and reset your
game.  Then load the saved game and you will start the level over
with the orbs you collected.  Do it as many times as you like. 
The only catch is it will lower your grade once you complete the
level.