I recently acquired the Lihit Lab Teffa Bag in the A5 size from Jet Pens. It comfortably fits my medium Ecosystem Life Artist notebook (5.25×8.25”) with room to toss bits of paper, letters and other items and still successfully zip it closed.
The outside of the bag has two velcro pockets, both gusseted; one comfortably fits a mobile phone, the other holds business cards, stamps and other tidbits. Behind each velcro pocket is a slit pocket to keep more goodies and on the far left is a pen pocket. I pack this thing to bulging and the 2-sided zipper makes it easy to close even cram-packed.
Available for $14 in lime green, orange, black, and brown. There is also a larger size to hold A4 for $18.50.