If you’ll excuse me for tooting my own horn and pointing out something you may have already seen, I wrote a blog post related to this a couple months ago. Wikipedia and the associated Wikimedia Commons are wonderful troves of photos, charts, graphs, maps, and other visuals.
When I teach US history I always make a point of spending at least a day or so discussing the Declaration of Independence, especially the “we hold these truths to be self-evident” part near the beginning. It’s not really that long, nor is it too hard for 8th graders to understand with a little guidance. Invite students to consider the Declaration’s continued impact on the American mindset, and to compare it to biblical teaching. You can also point out how the list of grievances towards the end of the Declaration are references to specific events.
Page Smith’s eight-volume A People’s History of the United States is my go-to American history browsing book. Smith dwells frequently on the ordinary person’s experience of the past. Edwin Tunis’s books such as Frontier Living and The Tavern at the Ferry are similar in this way, and you can share their beautiful illustrations with your students. If you can’t find these books at the library, they are easily available online at very reasonable prices (like $4 per volume, shipping included). Check bookfinder.com for the best deal.