This is a great question. When it comes to sources, there are many obstacles. And as a rule of thumb, it is much easier to find secondary sources than primary ones in nearly all subjects, including organizational theories. In light of this, let me make a few points.
First, there are more secondary material out there than primary ones. For this reason alone, it is easier to come across secondary sources. The reason for this is that primary research is harder to do than to write about the work that others have done. After the primary research is done, there are many scholars who chime in and the bibliography grows rather quickly.
Second, often primary research is older and definitely more academic in nature. Hence, these books are limited in print and often out of print. Hence, you will have to go to the library stacks to find them. As you can imagine, publishing companies do not make much money from primary research.
If you want to follow up on primary research, I suggest that you look for the most recent book and follow the bibliography. For example, take a look at: Organizational Theory, design and Changeby Gareth Jones. It went through six reprints, which shows that it is a popular book.
Primary sources are written by someone who has survived a war or experienced first hand, what you are writing about. Primary sources can be found in museums, historical societies, etc. In general it takes a lot of time and resources to find primary reasearch. Newpaper archieves can often take you in the right direction. Remember that war veterans, especially from WWI are few.
Secondary sources can be found in libraries, on the internet, in magazines, etc. Secondary sources are written about the subject. Secondary sources are much easier to find, however research the author of the source to be sure that they are an expert on the topic.