You would not go to a tax lawyer for a divorce or an orthopedic surgeon for an earache. It is the same with private investigation. You would pick one that specialized in accident investigation if you were disputing car wreck. If you need to find people and assets, you should pick an investigator that specializes in that area. If it is a criminal matter there may be different skills involved such as forensics.
Some agencies specialize in surveillance.
There are even investigators that specialize in computer crimes and tracking emails. It is a huge dynamic field with many skills and talents derived from other occupational fields as well. Begin your research by visiting the Legal Corner on this web site. I would be glad to help you with your case or refer you to someone who can. The initial consultation is always FREE .