https://www.uline.ca/Product/Detail/S-5614/Stayflats-Mailers/Kraft-Self-Seal-Stayflats-10-7-x-9?pricode=YD906&gadtype=pla&id=S-5

You should be able to find them in places like Staples, etc. They work. One thing to avoid completely is bubble wrap mailers. They aren't worth the money.