I'm trying to find out more about nutritional supplements for body
building, but am having a hard time finding unbiased sources of
information. I don't want any information from any site that sells
anything. I'm looking for some good ways to exclude such sites from my
search results. This is something I'd like to be able to do in
general, actually, and not just with regard to supplements.

One way to exclude commercial sites would be to limit your search
results to educational or government sites.

Here, as an example, is a search for "creatine," limited to sites
whose domain name ends in .edu:

://www.google.com/search?hl=en&q=creatine+site%3A.edu

That's helpful, pinkfreud. Thanks.