Surprise Bag

From MLB Power Pros Wiki

Can be acquired from Annie. Will give various items. A popular technique is to save before using then and quiting without saving until it gives the desired item (i.e. Gambling Ring or Secrets of Pitching/Hitting/Defense.

It can also be bought from the store during the final tweaking of your character, received during the Messy Room random event, or received from completing the Professor's Homework.