Any elder will do, at his home address. I assume that at least one of them is listed in Infospace.com.
*** km 1/06 p. 7 Announcements ***As a reminder, the return address of the branch office should never be used on any personal correspondence. This includes all letters and items of literature that are mailed to give a witness to interested people, to not-at-homes, or to those who live in inaccessible places. A publisher may wish to use his personal return address, since his ministry involves his personal service to God. (Matt. 28:19, 20; Rom. 10:14, 15) However, if he feels that it would be unwise to use his own return address when witnessing by letter, he may use his name and the return address of the Kingdom Hall.—See the May 2002 OurKingdomMinistry Question Box.
I wonder if this is the WTS directive, how a publisher could use the return address of the Kingdom Hall if no mail is received there?