For one book at least (Papadimitriou - born in Athens), you might consider writing to the author himself at Berkeley. He might have suggestions for some others, as well. You might even consider pointing out the problem in general and see if he might have ideas for a local solution for you and others at your university.
I once gave several boxes of books to a person who was paying to have them shipped to Kenya where there is a similar issue. Some US academics might have copies of older books. Many retiring academics might be willing to give up parts of their library, as I was. Shipping cost is a problem, however.
Some faculty members even periodically put out a box of books for students to grab. If you can somehow make a connection to them or their universities it is possible something could be worked out.
Some publishers will respond to requests for inexpensive (even free) copies of some books. It needn't be a Greek publisher. And some have subsidiaries that print lower cost versions various places around the world. These are priced more in line with local economies (though still expensive).
Living authors might respond to a request for a pdf with a promise of not passing it on. Their license from a publisher might permit it. Some have been given a few free copies of their work.
Professors in your country who can obtain grant money might (possibly) be able to get funding for books that they can lend or give to students. You and other students might try to work with the university to try to provide a solution. Money is always an issue, of course, but an appeal to a publisher from a university might be possible. And, lobbying to increase library funding is worth the effort.
Used copies in general are available, though, again, shipping costs are high.
Even in the US, the cost of books is a burden to students. This is partly due to the small market for advanced books that are expensive to produce.
Note that none of the above are especially likely to succeed. It is a problem throughout the world.