Private newsletters are those that are distributed just to the members of a group or association. For example, a university alumni association distributes its newsletter to all graduates of that university. Though it may seem that these would not be available to the public, they actually can be easy to find. They may be referred to as private newsletters, but that in no way means that the general public is excluded from reading them. Because such newsletters are generally produced on a tight budget by the sponsoring organization, and may even be published on a volunteer basis, there is no funding for a large distribution. Members of the organization are obviously the most interested parties, so they are the ones who get the newsletters. But in most cases the organization can provide an extra copy to any person who requests it, especially if it is for academic purposes.
Finding Out What Is Available
Private newsletters may be easy to find simply because the organizations that produce them are so narrowly defined. The newsletters themselves won't be listed anywhere. You have to contact the groups to ask if they do in fact produce a newsletter and what types of information are included in the newsletter. This last point is important. You only want to pursue a newsletter that will provide relevant information that you haven't uncovered elsewhere. You have no need for a newsletter that covers only trivial information, information that you already have, or information unrelated to your research topic.
How to Obtain Them
Once you have determined that a specific group publishes a newsletter that could help you, call or write to request what you want. If you have already made contact with someone from the group personally, request the information from that person. Otherwise, send a letter, which is less likely to be ignored. Let the group know whether you want copies of previous newsletters, or want to be put on its distribution list. Of course, if the newsletter is only published quarterly, you may not receive one before your paper is due!