A few ideas and items you might want to consider...

Try this article - Pros & Cons Of Online Directory Advertising for Process Servers and Private Investigators - "There are a lot of directories online that profess to charge a modest monthly or annual fee to list a Process Server or Private investigator, and they insist the investment will produce results. Here's my personal feelings about the value of Directory Advertising, and what you should think about when making this decision:" - FULL TEXT

Or expand your library with these books -

Or try some case management software - PI - Professional Investigator (TM) - Private Investigator ... Case and contact management software for PI's, investigators, detectives, bailiffs.