The proposed Constitution was presented to the people for debate prior to it’s ratification. It was vigorously debated in a flurry of pamphlets, many of which have come down to us. Some of these can be accessed online for free.
Another source for pamphlets about the Constitution can be found at the Online Forum for Liberty. They have other useful resources as well.
The Online Library of Liberty also provides a nice compendium of resources.
U.S. Constitution Online provides not only information about the Constitution but also resources to teach it, including to kids at various levels.
Constitution Facts . com offers not only the text of the Constitution and the Amendments, but the Declaration of Independence, the Articles of Confederation, the Founding Fathers and more. They also sell pocket editions of the Constitution, including a free copy for individuals, but you must pay shipping.