When historians talk about evidence, they often mean primary sources. Primary sources (diaries, letters, speeches, newspaper accounts, artifacts, etc. created during the time period being studied) can offer rich and unique insights into your topic. Thousands of primary documents have been digitized and can be found online on university, government and museum websites. The Bentley Library also has collections of these documents in printed source books.
U.S. History in Context (Gale) - The Gilded Age (includes books, journal articles, primary sources, images, and more!)
The Gilded Age and Progressive Era Reference Library (full text, searchable book from Gale with many primary sources)
Interstate Commerce Act a secondary AND primary source from Issues & Controversies in American History (Infobase)
The Gilded Age overview article from The Encyclopedia of the Age of Revolution and Empire (1750-1900)
Platform of the Populists from Social Policy: Essential Primary Sources