Depends where you live? I've had good service from this seller in the past.

Alternatively if you need a broader selection, there's jd.com. You have to do everything in Chinese, though, if you want to order books from them. (They have an English site but it's just electronics.) I can give more advice on jd.com if needed.

There are also stores like purpleculture.net, but they tend to have limited selection.