These are really nice because they add functionality to your traveler’s notebook if you wish to use it as a wallet. You just slip these under the elastics as you would an insert booklet, and they hold credit cards, coins, notes etc.
I have one from Monique at Morgan Le Fae’s Trinkets.
Here are some more options available in different sizes from Monique.
Jerome Phillippe also has some in his Etsy store (available in different sizes) – this time with a zipped pocket for coins.
You can of course find plastic pouches on Etsy that fulfill the same function. They come in several different sizes, such as the ones below from PaperGeekCo.