I am currently looking for some websites that can provide useful historical information, digitilised newspapers, articles, treaties. Academia.edu is the only source I know and its limited to some vague articles, and I am not really familiar with its Search engine. Any proposals?
I am already looking into libraries for books familiar to my subject (the reasons that led to the rise of the 1936 dictatorship in greece) but I need more.
There are a couple that I can think of off the top of my head. The GWU National Security archives is a good place for documents relating to the US acquired via the Freedom of Information Act. I used that place religiously in a Latin American history course back as an undergrad. Some documents are still redacted since FOIA cannot compel the government to release certain kinds of classified information, but it still has a LOT of good stuff.
JSTOR is another good resource but it usually requires institutional access for large swathes of its collection. It contains a lot of journals, journal articles, some digitized primary sources/pictures, book chapters and reviews, and even complete books but your level of access to these items will vary considerably. I am lucky enough to have full university access through my university alumni resources and maybe you'll be similarly lucky. Otherwise, it can be a bit tricky to access. Similarly, there are other online databases for journals (TandF is a big one) but these places can be quite difficult to access without the help of an academic or corporate institution.
Lastly, if you are a current university student, you can always ask your school's librarian for help. A lot of academic libraries have some sort of collection going on and maybe they'll have digitized items or collections which have been vetted beforehand. If not, then there are some school libraries with freely accessible digital archives of their own collections which might be of interest, like this one. Best of luck.
As above,national library or large regional libraries will have most of what you need. They may not let you touch books older than certain age but will have them digitalized or microfilmed. Local an national archives are a very good source too, tho access may be slightly more difficult, but with a letter from your Uni you should be fine. Of course I can't talk about Greece, but sure it doesn't differ much from where I am. Be prepared tho - it is time consuming exercise. And as guys/gals above - ask your librarians, they will be able to explain how system works or even arrange access "remotely" if possible.