I'm willing to give a mathematics Olympiad [syllabus] and I can't buy all the book(or even some of them) and am looking for some online resources like questions, articles, or more prefably some single book with all topics(least probable).Or I would be pleased even if someone suggested some places to look upon for high level of questions for practice, note that you needn't mention this olympiad's(IMO's) previous questions as I would be doing them either way.
You can try to look at the Art of Problem Solving website here. It has tons of problems with discussions, a lot of materials on different topics et.c.
There is a very nice book by Prasolov with problems in geometry which I would also recommend. It has a gigantic collection of problems, and a lot of them are pretty complicated.
I hope this helps a bit.