Navigate to the "Extras" menu, then select "Codes". Enter the following codes to unlock the the following costumes for purchase.

Effect	Password
Daphne Karate Black Belt costume	2839
Fred Knight costume	4826
Scooby-Doo Egyptian costume	1585
Shaggy Native American costume	3726
Unlock All Levels	2129
Velma Pirate outfit	6588