Short answer: reading, discussing, pondering, and analyzing. The "pondering" is where you tap into your inner reservoir of knowledge or receive inspiration from the 'muses'.
So to answer your questions regarding my own case, I get this either from discussions I have with friends, intuitive 'thought balls' that pop into my mind when I go about my day, or answers that I come up with when contemplating issues and sketching them out in my notebook.
I make the most progress by sitting down with my sketchbook and pondering a question or paradox until various answers come through - then I shoot down the ones that have logical holes or fail to explain the issue in question, and build upon the rest with a further round of questions. In time a framework fleshes itself out that matches all I know up to that point, connects some things I couldn't connect before, and suggests some possibilities that later are either confirmed or contradicted by further info I come across...the contradiction creates a paradox and then it's back to the sketchbook.
Generally what I come up with (or tap into) I later read about in some book or article, and I try to reference these in my own articles as independent backup or elaboration but not necessarily as "source" material. The one thing scholars and intellectuals don't understand is that we all have the truth within us, and writing through this faculty of inner realization generates something not strictly a patchwork of "what we have read before" and thus not all of it can be referenced.
A couple times in college papers I included a paragraph of my own thoughts on an issue presenting some idea I hadn't read anywhere but which fit perfectly into the discussion, and was red-inked for not citing my sources in that paragraph. So people in college are trained to be parrots saying only what others have said, but what else is new.
On top of intuitive reception, info can also be gleaned from observing one's own life and that of others. Together, people's personal experiences go a long way in revealing the fundamental principles of 3D/4D living. Many books are written via the author interviewing or working with many people and then forming a hypothesis based on what they say. Other books are written based on personal experience. But overall, real progress can only be made if one starts with a source of info whether it's a book, article, personal experience, annecdotes by others, etc... and then grinding this info through the internal "eye" of understanding or realization and thereby generating something that transcends the sum of all this info.
Acquiring fringe knowledge is like digging for diamonds in a mine field.
