To find the Professor Quirrell (Voldemort) character token, you must enter Year 1, Chapter 6 - Face of the Enemy in Free Play. Once you reach the room with the chess board, look on the left side of the room for a dark magic object. Destroy this object and you'll obtain the character token.
To find the Tom Riddle character token, you must enter Year 2, Chapter 6 - The Basilisk in Free Play. Once you reach the boss battle, there will be a safe in the top-right part of the room. Use Griphook (who you can obtain in Year 1, Chapter 1 inside the vault on the right at the end of the level) to open the safe. Inside the safe is the character token.
To find the Lord Voldemort character token, you must first get all 200 Gold Bricks. After this, go to Borgin and Burkes in Knockturn Alley (you must use Reducto on the gates in Diagon Alley to access Knockturn Alley). Inside, you will be able to put together the entrance for the Bonus Level. Upon completion, you will automatically receive the character token for Lord Voldemort.
After you've obtained all three character tokens, don't forget to go to Madam Malkin's to purchase them!
If you're having trouble finding them on the character list, Lord Voldemort and Professor Quirrell (Voldemort) are in between James Potter (Ghost) and Lucius Malfoy and Tom Riddle is in between Lucius Malfoy and Peter Pettigrew.
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