ok... didn't mean to go off like this, but finding good bookbinding paper is something that occupies way more of my time than it should. Really, why is it so hard to find something nice?? I feel your pain. Here is my advice.
if you live near a Michael's or a Hobby Lobby (or similar) you can get large sheets of paper (usually about 19 x 25 inches) that is a nice quality... suitable for drawing or papercrafts, but not as heavy as cardstock. Usually $1.15 - $2.50 for what I pick up (Drawing paper examples: Canson Mi Tientes I think? Also Fabriano Tiziano). You just cut it down to the size you need. All different colors, sometimes with flower petals or threads in it, and usually different weights are available too. Kind of nice because you can tear the edges with a butterknife if you want kind of feathery edges, or you can make really sharp cuts with some kind of blade.
These oversized sheets are ALSO nice because you can make sure the grain of the paper goes in the right direction. Take a sheet of paper -- any sheet not made by hand -- and bend it so that the top and bottom are together. (Don't fold/crease it, just bend.) Now unbend, and bend the two sides toward each other. See how it's easier to bend in one direction than the other? That's the way the grain runs, and if you utilize that natural quality of the paper, your bookbinding will be easier and neater, and will resist warping over time. (Most types of office paper have a grain that runs parallel with the longer edge; i.e., hot-dog bend is easier than hamburger bend, haha) ::Phew:: Sorry for that tangent!
*If you DO end up using cardstock, try getting coverstock instead, as it folds a little easier, try folding with the grain direction, instead of against (perpendicular to) it, and definitely use a bone folder, or at least the bowl of a spoon, to help you make sharp creases and avoid that cracking effect.
Back to paper sources. I have my eye on some paper at this site: www.paper-papers.com
. They offer lots of nice paper in "Full Sheet" (~19 x 25 inches) size, and they will ship it that size or cut it down for you. This is kind of pricey though.
Also check out bookbinding shops like www.hollanders.com
-- I am partial to these people because they are an awesome local business, very helpful and knowledgeable! -- here's a link to their papers: http://www.hollanders.com/decorative_papers.htm
scroll down to the last category and check out some of the offerings.
If you are looking online, search for text-weight papers, and choose something in the 70-100 gsm range. 70 gsm is like a nice writing paper; 100 gsm is nearing cardstock thickness, but not as stiff. (These would be similar to a 30 - 40 lb paper, if that helps.)
I guess I would mention the full sheets at Michael's/Hobby Lobby again, or else pads of drawing or sketch paper. Pastel paper is nice because it's heavy, but not so heavy it doesn't want to fold.
Sorry this was so long... I am a little bit of a paper whore.
Hope it helped a little.