One underappreciated resource is online databases at the library. As a resident of your state, you can get access to any library in your state and not just whatever is local. Not all libraries have same information, but larger cities have very interesting resources. You can check on their websites without entering a card number. In California, SFPL has full Alldata access, for example.
Visitors to NYC can get a visitor library card valid for 3 months. That accesses over 600 resources. This may be true for other major cities and large counties.
There are “unofficial” websites that sell service manuals. I bought a PDF copy of my van’s manual, which is 1399 pages. My concern was wiring diagrams and torque ratings.