Do you have their names and a last know location? Not such a daft question as some might think. There are sites that might help you in your search, obviously you could post names, last know addresses, places of work or schools on this site or the Friends and Family Tree sites. There is also the option of placing an ad in the personal section of the local paper. This is one site that helped me when I was looking
www.192.com. But be prepared for the fact that they might not want to know, which to my deepest regret I found.
Good luck.......