This is probably the easiest of my supplies - for most everything I paint, I use Arches 140lb coldpress paper.
I prefer it in block form - which means that the paper is gummed around the edges, creating a 'block' of paper. This helps (though, does not completely prevent) the paper from warping.
I typically paint pretty small, so my favorite sizes are anything less than 12 x 16. Recently, I found out that Arches created a square format block - it's my favorite! This is the only place I can find it online.
I remember the first time I doled out the money for a block of this paper. I was beyond excited to have 'REAL' artists paper... I know I'm making myself sound ancient here, but back in the old days, there was no such thing as youtube to help me figure out how the heck to get the pages off the block. I mangled it. I cried and cussed, like a lot. When I finally figured it out, I felt like a complete nincompoop. So I created this video to help you not feel like that. The struggle was real.
I have a few art journals (if a few means too many). I've tried all different brands, but my favorite is the Global Art Materials 'HandBook' Field Watercolor Artist Journal. Available in many different sizes. The paper is good and I prefer spiral bound books as I have no problem ripping out a page I am not satisfied with or one that I am completely satisfied with and want to scan. If you prefer non-spiral, they make that too - you can find it here.