However, it is less likely that you will work with someone you know, and also less likely that you will work on a project that matches your research interests exactly. However, the benefit of this strategy is that you can work on a project that is of direct interest to you. If you are going to write your own postdoc, it is essential that you start making contacts in the year or two before you graduate from graduate school. This means that you should single someone out that you would like to work with. You may either know this person already, or you may know of their work.
Professional conferences are a great way to meet prospective collaborators.
You might also consider having th person be an outside dissertation committee member. To write your own postdoc, you will need to get information regarding available grants. For advertised positions, there are several sources that a prospective postdoc can use. Quite commonly, job and postdoc announcements are sent to department Chairpersons, who will then post them. In addition, a variety of academic and scientific journals, including Science, New Scientist, Chronicle of Higher Education, and Animal Behavior routinely advertise postdoctoral positions in biology and zoology.