Personally, I like the ones from handbook journal co they have watercolor paper or a less expensive version depending on what you're planning on using it for primarily.

I've heard good things about illio but haven't tried them myself.

You're also usually going to be safe with any of the canson, strathmore or hahnemuhle mixed media or watercolor paper. I'm partial to the Strathmore softcover one myself. But I will warn you, the softcover gets dings and stuff super easily, so if you want it to stay "pretty" while you carry it around, it's not going to be for you.