Found out that it's not that easy to buy a quality sketchbook: the paper might be too thin, it may be uncomfortable to use, or you may run out of pages at the worst possible moment.
I found a solution: it's just an A5 binder. I'm cutting in half A4 draft paper (it's hard to find thick and dense a5 paper).
Each file in this folder is a separate sheet - so it's easier to work on, and if you screwed up - just get another sheet: you insert it into a folder only if you liked it.
The only downside of this method - the folder is getting thick really quickly: comfortable thickness is around 30-40 pages.
Upside is: you can have two folders: one is for carrying around, and the other (high capacity) for storing your work.