How does an executor find a heir if they have no address?

For probating the estate, the executor is usually instructed to send notice to the last know address and to publish a notice in the local newspaper.

However, as a practical matter, there are people who do that type of research. Also, you should ask family members, distant relatives, old friends, neighbors, former employers, class reunion organizers, etc., to see if anyone has any idea where he might live. You can search online and at websites such as Ancestry.com. Probate lawyers usually have some experience and associates who can do more thorough search.