Finding secondary resources is increasingly a matter of discipline and efficiency rather than stamina. For the scholar, the development of electronic finding aids has made the task of finding out what has been published far less time-consuming than in the past. Although the format may have changed you will still be using traditional finding aids to identify what has been published. These include reference tools (including bibliographies), abstracting/indexing services, recommendations from your professors, and even shelf-reading.
The tools you use to identify a primary source are essentially the same as those that you would use to identify ANY book or article relevant to your topic. Naturally, the full text primary source databases in this guide are handy for articles written during the time you are researching. Additionally, autobiographies, memoirs and collections of letters are generally published in book form. An excellent source for citations to primary sources is the bibliography at the end of a scholarly book or article. Just as a scholarly text lists the secondary sources that scholar used to support his or her thesis, that bibliography will also note all primary sources used. Many specialized reference sources also list primary materials.