I generally use A5 exercise books for various things – long lists, journalling, cutting out and pasting in things I would like to buy, story ideas (I write on fanfiction.net).
To keep them together I put them in these Snopake envelopes, which I have in three different sizes. These plastic envelopes are cheap, really hardy and fit A5 perfectly. They do, however, have very sharp corners which tend to cut into things.
I have always loved Zenok Leather on Etsy but I already have a Midori passport size, a full size, and a Raydori so always just lusted from afar.
Today I saw that he did indeed make A5 notebooks and in gorgeous natural beige leather. So have ordered one to use instead of my Snopake envelopes. I absolutely cannot wait. I have seen so many of his Zendoris and I love them.