What secondary sources could I use that relate to The Grapes of Wrath?
Well, a good starting point would be to have a look at the movie version of this book if you can get a chance to see it. It was adapted into a film that came out in 1940 and this adaption was actually very successful, as it won various awards that proved its worth as a film of this time.
Another place to go to would be other texts that are set during the Great Depression and that deal exclusively with the impact of this time on the citizens of America. One of my favourite texts that I think compliments this novel very well is actually a text designed for young adults, called Bud, not Buddy by Christopher Paul Curtis. This tells the story of one young African-American boy's search for both his father and his own identity against the backdrop of an America that is plunged into the recession.
You might also want to google image the Great Depression and see if you can find various images of people impacted by it.