Max, here's a pretty comprehensive breakdown of the Median army's weaponry by Herodotus. Scroll down to "Equipment," though the rest of story is an intriguing read, too. Large bows are mentioned, but no references to their being made of metal. Found this by tracking down leads through the Archaeology Magazine site search. I may be back with more...

http://members.ozemail.com.au/~ancientpersia/org.html