The earliest that survive were found rolled up under the heads of mummified Greek Egyptians in the Egyptian deserts from about 150 to 200 B.C. But they're just fragments, not the whole Iliad. The oldest complete Iliad is a manuscript found in the doge's library in Venice. A French scholar discovered it at the end of the 18th century, which is why it's called the Venetus A. It had come to Venice from Constantinople-Byzantium, where it had probably been made in about A.D. 900, thousands and thousands of years after the poems had first been composed.
More importantly, it contained all kinds of marginal notes, the so-called scholia, which had been made by the great editors of The Iliad in the Greek city of Alexandria sometime between the first century B.C. and the first century A.D. So what you have in Venetus A is not only the text of The Iliad but also what these ancient commentators thought about it.
[buradan-->National Geographic Book Talk:
Author Says a Whole Culture—Not a Single 'Homer'—Wrote 'Iliad,' 'Odyssey']