These are the following variants of Harry that you will need, as well as their locations:
Harry Potter (Hogwarts uniform)-unlocked automatically
Harry (Pyjamas)-Found in the Gryffindor Common room (Throw darts at Malfoy's photo)
Harry (Winter)-found in Hogsmeade free roam (hit the seven icicles above the door)
Harry (Christmas)-found towards the end of level 6-2 (Pink Baseplate ability needed)
Harry (Albert Runcorn)-found at the beginning of level 7-2 (Bookcase character needed)
Harry (Godric's Hollow)-found towards the end of level 7-3 (Dark Magic Character needed)
Harry (Locket)-Found underwater in level 7-4 (Next to frozen sword of Godric Gryffindor)
Harry (Brown Jacket)-found towards the end of level 8-4 (Repair character needed)
You will also need Voldemort, this character token is found at the back of the last Area of level 8-6 behind a broken archway.
Once you've collected all of the characters needed, you can simply go to The Leaky Cauldron hub and use the polyjuice potion to have one Harry and one Voldemort. Kill that variant, and then continue to use the polyjuice potion to switch to a different Harry. Make sure you have Voldemort's spell wheel set to Green so that it keeps destroying the other character!
EDIT: You will also need the Yul-Ball Version of Harry as well!