Full list and locations:
Harry (Pyjamas): Gryffindor common room - darts
Ginny (Pyjamas): Gryffindor common room - hitting the painting next to the exit
Ron (Blue Pyjamas): Bookcase outside the Hufflepuff common room
George (Pyjamas): Weasley's Wizard Shop after beginning Deathly Hallows Pt1
Fred (Pyjamas): Weasley's Wizard Shop after beginning Deathly Hallows Pt1
McGonnagal (Pyjamas): Gryffindor Common Room. Use the Deluminator to take the light by the door over to the painting on the left side of the room
Neville (Pyjamas): Divination Room after assembling the Giant cup of the the broken smaller ones
Slughorn (Pajamas): Located in "Out of Retirement" for destroying the 5 gnomes (all before you go in the house): One is next to the fountain, another all the way at the bottom of the road in front of a gate, a third on top of the garage, the fourth is in a key vault behind dark magic, and the last one is behind the other dark magic gate.
You must unlock the token AND purchase it at Madame Malkin's shop to get the achievement.
I take absolutely no credit for the list; thanks to WaHammy's guide on Xbox360achievements.org for the information.