What does "Scour" mean?

It means "go through, thoroughly" (the book shelves.)

Do you mean that "Scour the “non-mainstream economists” section for some of the most popular ones" refers to "the shelves of the “non-mainstream economists” section are full of some of the most popular books"?

If so, " the most popular books" is logically referred to "mainstream economists' books". It's a contradiction.

Popularity doesn't always equate to 'mainstream'. We have some quite popular "news" tabloids sold at supermarket checkout stands, dealing with alien abductions, and so forth. While popular, they are hardly mainstream news sources.

Back to scour. Scouring is a cleaning process involving free abrasive grains. It is a very thorough cleaning technique. Now, the word is used more widely.

Thank you Roger.

While your reply was informative for me, I still failed to get "Scour the “non-mainstream economists” section for some of the most popular ones". Please rewrite it in more details.

mainstream economists are those who share their economic views with the majority of fellow economists. It's not who sells the most books to the general public, or, in this case, a small segment of the public. For example, mainstream economists believe that demand drives supply--the more demand there is for a product, the more will be produced. Supply-side economists, the darlings of the
far right, say that if you supply more product, then the demand for it will increase. That is highly questionable. They are non-mainstream because it's a minority view in the community of economists. From the context, the non-mainstream the author is talking about are economists with a marxist or communist perspective, not the economists or pop economists who write things like how-to-succeed-in-business books which are the staples of thje economics section in Western bookstores.

If you scour, i.e. look very thoroighly through, the non-mainstream authors section, you'll come across some of the ones the store sells the most of. It's a bookstore where Mao's spirit is still the driving force.

Most likely any economist who does not buy into globalism, cutting labor costs, encouraging ever increasing levels of consumption, or who does not focus on creating ever increasing paper wealth is going to be considered a "non-mainstream economist". It takes very little to get branded a heretic in that business.