You can certainly get all the information online if you know where to look and what you need. First, you'll need to go through the ACS and cross-reference all the references to the areas/tasks. This will point you to the Pilot's Handbook of Aeronautical Knowledge, the FAA Risk Management Handbook, the FAA Weather Handbook, the AIM, various ACs and SAFOs, and finally, the full list of regulations. The ACS will give you the books, but it doesn't tell you which chapters or sections of chapters are pertinent. For example, only about 20% of the Pilot's Handbook (PHAK) is relevant to Part 107 issues. Without guidance, you have no idea what to actually study. The same thing with the rest of the books and docs.