I've done plenty of life drawing in whatever cheap smaller sketchbook (approx A5) I could find at the nearest art store. The paper isn't really made for paint, but I made it work. They're nice because they can fit in a big jacket pocket or a bag, and you can draw while standing, even if there's a crowd.
Another option is to get a clipboard in fill it with printer paper. Cheap, easy, and it takes pencils and pens well. And if you're not happy with a drawing, you can just toss it in the recycling.