Q103. "I have heard that the U.S. Constitution is based on a document called the 'Iroquois Confederation'. Is this true, and if so what parts of the Constitution came from this document?"
A. The U.S. Constitution is not based on that of the Iroquois Confederation — but some of the delegates to the Convention knew of the Confederation (notably Benjamin Franklin) and the two documents share many concepts, as do many constitutions. The Five Nations was a federal-style system, and it shared that with the U.S. You can find it online, though it is slightly difficult reading — the Indians spoke often in concept and metaphor.
I found a copy here: The Tuscarora and Six Nations Website.