Here are five popular textbook publishers:
Holt, Rinehart & Winston
I know that McDougal-Little has a web site:
to which our teachers and students have password-only access. They can use the entire textbook there.
I went there and navigated to this page:
From there, I was able to access sample textbook pages. For example, here is the URL for a page from a World Cultures and Geography text:
If you go to http://www.boiseschools.org/
And click on Curriculum, you can navigate to the social studies curriculum for each grade level. The textbooks for each curriculum are listed there.