I believe the term is called Doxing. Is there any websites with a subscription to pay for a database look up to gather information to recover their own account from themselves. If it helps for specifics the the matter of topic is Runescape
There is no actual site that is good for doxing.
Best way is to find some info on them, maybe their email, username, go to a breach report site like haveibeenpwned, look up their info there if they are in some database leak, look up the leak, see if it contains other stuff about them.