The A5 size paper letter fits perfectly in three folds. It is a double envelope with thin paper that will be like when sending with a single sheet.
Kokuyo Century Edition Thin Paper Notepad A5: Dot Grid
Kokuyo Century Edition Thin Paper Notepad A5: Plain
Omoriyuko Letter Set: Shiro Sumire (Motoko Omori)
Oeda Letterpress Noble Message Pad (Pale Blue/Pale Green/Pale Pink)
Miki Tamura Letter Set: Write a Letter