I like the Hahnemuhle and the Stillman & Birn Beta or Deltas, though both perform quite differently. And that leads to the question. Other sketchbooks that I like are the Global Arts watercolor handbooks.
Do you have a very definite preference in the paper you currently use? What I like might not suit you at all.
Usually paper in a sketchbook is a bit thinner, which means more problems with curling, buckling and dimpling. I also find you get more harder edges. You need to stay with lighter washes, which means wet-into-wet techniques are not as easy to use.
You might try making your own sketchbook to begin with. A pamphlet stitch is very easy, and there are all sorts of instructions to be found by googling it. I’d recommend starting with a small book, maybe five sheets of paper, large enough to fold in half to the size you want which would give you 10 pages, 20 if you use both sides. For this size, I don’t always add covers, but if you want them you can cut cereal boxes or cardboard to size and glue them to the front and back sheets.
The advantage here, beyond using the paper you are used to, is that you can see if you like painting in a bound book before paying for one and can use paper you’ve already bought.
An alternative is to just paint on the paper you normally use but tape off one edge. I took the cardboard backing from a couple of paper pads, covered them with newspaper and painted them. Then I punched holes in them and inserted rings that can be reopened. As I finished a painting, I punched holes in the unpainted area that I had taped off and added it in. Sometimes, I punch and add all the paper before hand and then paint right from the sketchbook.
