I’ve used exclusively Design.y for the last decade. Hand-bound leather notebooks from Sendai, Japan, they use only Tomoe River paper. Ultra-thin sheets with no bleed through; the best paper in the most beautiful wrapping. I find the EP Record (A5; plain sheets) to simply be the best notebook in the world. You can find the (typically awful) website here.
http://design-y.near-mint.com/
