For the period of classical Greek mathematics (600-150 B.C.), there are a very few types of source material:

• mathematics books and papers of Euclid, Archimedes, and Apollonius and a few lesser authors that have survived, sometimes going through several different languages (Greek, Syriac, Persian, Arabic, Latin) and several hundred years before reaching us;
• some brief references in the works of Plato and Aristotle (Plato's items are clear and interesting if short, but I myself cannot make sense of Aristotle);
• a tiny number of nearly contemporary artefacts on papyrus and bits of pottery;
• fragments of lost works quoted by later authors mostly from Roman and Byzantine Egypt, mostly mathematicians or philosophers, and all writing several hundred years after the period they are writing about;
• commentaries by the same authors on earlier mathematics;
• even later Arabic and Byzantine authors, now writing about 1,000 years after the period they are writing about.