To make every item available at the in-game store, insert the following lengthy cheat at the in-game cheats menu. Keep in mind that because you're strumming using four buttons, no sounds will play. You'll have to check the store when you're done inserting the password to see if it worked:

(GR_BO) (GRYB_) (GRY_O) (G_BYO) (GRYB_) (_RYBO) (GRYB_) (G_YBO) (GRYB_) (GRY_O) (GRY_O) (GRYB_) (GRY_O)