Nowadays Chinese are getting really good at making LEGO-compatible blocks. Some sets are really unique and of very good quality. But still - we got our own ideas. I wonder does anyone here know any source in China, that produces blocks (not sets)? So blocks of requested shape and color. I would like to contact them somehow. They should exist, since set producing companies do not seem to produce blocks themselves, but rather taking them from somewhere else.
Note that these don't appear to be significantly better prices than what you might find on a secondary LEGO market such as Bricklink, and quality varies a lot between brands, so I'd recommend sticking with Bricklink unless you are looking for massive quantities and quality isn't as important to you.
High quality LEGO compatible brick manufacturing requires very high mold tolerance and tight tolerances on the entire injection molding process. It isn't easy to do cheaply, and LEGO's scale allows them to be fairly price competitive on bulk parts among manufactures with similar quality.