Below are the locations of the music discs in Chapters 5, 6 & 7 of Final Fantasy VII Remake!
Chapter 5
28. Stamp
Purchase for 50 gil from a vending machine in Section E of Corkscrew Tunnel.
Chapter 6
18. Electric de Chocobo
Purchase for 50 gil from a vending machine at Sector 4 Plate Interior: Plate Mid-Level in Section H Utility Access. To reach it face the cargo platform that you had to power up during the story, turn right and head west. Take the lift across to reach a platform with stairs leading to a room containing the vending machine.
Chapter 7
2. Bombing Mission
Purchase for 50 gil from a vending machine near the Front Gate in the Mako Reactor 5. This will be right after the segment where Barret, Tifa and Cloud have to move levers to unlock a door.