It depends upon the laws of the country and the person'e status or wealth. In Australia you can buy a copy of the electoral roll - the names of all voters. It gives phone numbers and addresses if I remember correctly. Vast amounts of data is stored in councils (for rates etc), Government (tax data, vehicle registration etc). I believe, for a fee, it's possible to enquire about births, marriages and deaths - my wife has paid for copies of death certificates of near relatives. You can walk into a sporting club and casually ask if Joe Blow is a member if you think you know the rough area of search and any sporting interests of the victim. If secrecy is not important, ads in the classified section of main newspapers under "missing person". Membership of clubs, address his old University sends Varsity News and other correspondence.
Then there are tricks. An official-looking ad in the main paper stating a prize has not been claimed - naming the missing person. It would have to be an attractive amount (though you wouldn't have to have the money) and a feasible competition (Lotto in Australia would be a suggestion).
The main thing you need for good searching is the date of birth. This cuts the field down, even for John Smiths. And if a person doesn't want to be found they often change their name slightly. John James Smith goes by James Smith. And if you're looking for Alexander also look for Alec, Alex, Lex, Sandy, Alejandro, Jano - all the usual shortening of the names. You can dedicate a whole chapter to the way a person will change their name so they won't get it wrong (it would have to be near the real thing unless the person is a professional criminal). People also change their DOB by swapping month and day - 1/11/44 tp 11/1/44. This is so they can claim it was a pure mistake if they're caught.
So work on you plot backward. Think up how much the victim DOESN'T want to be found. Once you determine that, work out the sly ways he/she alters personal details. Then you'll know what the searcher will have to do to find the person.