There are many ways to locate witnesses and suspects. Some of them are best left to an experienced cold case organization, law enforcement, or investigators. Ohers are safe and achievable by civilians.
Facebook is still a leading method of tracking individuals. (And remember that, while younger folks may gravitate to new social media platforms, many people involved in cold cases are at an age where Facebook remains their primary medium.) You can search for pages on Facebook, Instagram, etc., by individual name. If you know a friend or relative, you may be able to look at their friends or comments on their page, especially on posts at meaningful days, such as a birthday or marriage. We advise against friending witnesses and suspects unless you already have a natural connection to them.
The most powerful location database is only available to law enforcement, investigators, and attorneys.
Some relatively inexpensive databases such as TruthFinder or InstantCheckmate are pretty good at tracking or confirming the identity of individuals and their relatives.
if you have an existing relationship with friends or family of the witness/suspect, there may be a natural opportunity at which the subject of what they're up to these days comes up. Otherwise, be careful; remember that blood is thicker than water.
People interact with courts in a wide range of areas, including minor things like traffic citations. Many court records not only contain dates of birth and middle names (to avoid confusion over similarly named individuals), but also the person's address. In some criminal cases, a defendant wanting a court-appointed attorney has to complete a form that includes his last address and place of employment.