i've done amazing things finding people online.
recently, i located a childhood friend i moved away from when i was 6. all i remembered was her first name and the house she lived in when we were friends. it was also difficult because even if i knew her last name, her mother and step-father had a different surname.
it took a few hours but i eventually located her using Google Maps (to find her house and street number), free public records (to locate the name of the last ownership of the home), obituaries (which located me her maiden name and her now married name), myspace (which thankfully had photos of her even though she'd aged quite substantially) and finally facebook (which i used to contact her).
in your spot though, i'd think their family or friends would be the best sources. if the family is refusing you the contact information, perhaps they think they have a good reason...? could you write the person a letter and give it to one of the family members? clearly they may not deliver it but maybe... at the very least it may be cathartic for you to put what you have to say to paper and then place it in the proper hands. at that point though, i think you'd have to be satisfied that you've truly done all that YOU can do, y'know?