Yes, stiff felt sheets! Found in crafting sections alongside its close cousin, floppy felt.
tools Pop Goes the Page
How to Stiffen Felt: Unleashing Creativity with this Simple Trick
Here is how to make quiet book pages stiff
We Are Not Like Them, Book by Christine Pride, Jo Piazza, Official Publisher Page
Inside the Delirious Rise of 'Superfake' Handbags - The New York Times
Here is how to make quiet book pages stiff
Picture Book Projects, Pop Goes the Page
Stiffened Felt 4x6 Sheets
Get To Know The Many Sounds Of Asian Pop: From The Philippines' BGYO To Hong Kong's Tyson Yoshi & Thai Singer Phum Viphurit
Creep, Book by Myriam Gurba, Official Publisher Page
How to Stiffen Felt - Cutesy Crafts
How to Stiffen Felt: Unleashing Creativity with this Simple Trick
Make stiff felt sheets at home, super easy tutorial!