If you need some extra money while shopping in Diagon Alley, pay a visit to Gringotts to visit your vault.
Gringotts, a centuries-old bank run by goblins, is located on the north side of Diagon Alley. You will need to find the goblin who guards the key to open your vault - he is VERY close by.
The cache contains plenty of knuts, sickles and galleons for your purchases. Please ensure the vault is re-locked and the key returned.