Published collections of primary source documents are excellent resources for research papers in history.
Such collections are well-organized, with tables of contents and indexes to make it easy to locate relevant documents. The documents often contain helpful introductions and/or annotations to place them in context and clarify obscure references.
At the Cheng Library almost all of our published primary source collections in history are part of the circulating collection, so they can be borrowed while you are working on your paper.