This article is from Bridget O'Connor's blog. I used a pad of w/c paper when I took my painting trip to Italy and really liked it.
Here's a tip on how to make your own watercolor pad notebook.
Take a sheet of 140 lb CP watercolor paper and divide it into 10 equal pieces, then go to Staples or any local office supply store that makes copies with full services and have it made into a spiral-bound notebook.
Instead of spending a lot of money on purchasing a notebook from an Art store that may or may not have the kind of paper you want, get exactly what you needed. The cost is minimal for this service for instance these watercolor notebook only cost $ 1.50 for the cover and binding.
By making your own notebook you can create any size booklet that you want. This is a perfect idea for traveling, testing colors combinations or taking notes when going to workshops.