You can accomplish service of a person at a PO BOX/
A street address isn't required.
Here you go, have yourself a read.
Or, hire yourself a process server to complete the dirty deed.
415.20. (a) In lieu of personal delivery of a copy of the summons
and complaint to the person to be served as specified in Section
416.10, 416.20, 416.30, 416.40, or 416.50, a summons may be served by
leaving a copy of the summons and complaint during usual office
hours in his or her office or, if no physical address is known, at
his or her usual mailing address, other than a United States Postal
Service post office box, with the person who is apparently in charge
thereof, and by thereafter mailing a copy of the summons and
complaint by first-class mail, postage prepaid to the person to be
served at the place where a copy of the summons and complaint were
left. When service is effected by leaving a copy of the summons and
complaint at a mailing address, it shall be left with a person at
least 18 years of age, who shall be informed of the contents thereof.
Service of a summons in this manner is deemed complete on the 10th
day after the mailing.
(b) If a copy of the summons and complaint cannot with reasonable
diligence be personally delivered to the person to be served, as
specified in Section 416.60, 416.70, 416.80, or 416.90, a summons may
be served by leaving a copy of the summons and complaint at the
person's dwelling house, usual place of abode, usual place of
business, or usual mailing address other than a United States Postal
Service post office box, in the presence of a competent member of the
household or a person apparently in charge of his or her office,
place of business, or usual mailing address other than a United
States Postal Service post office box, at least 18 years of age, who
shall be informed of the contents thereof, and by thereafter mailing
a copy of the summons and of the complaint by first-class mail,
postage prepaid to the person to be served at the place where a copy
of the summons and complaint were left. Service of a summons in this
manner is deemed complete on the 10th day after the mailing.
http://www.leginfo.ca.gov/cgi-bin/displaycode?section=ccp&group=00001-01000&file=415.10-415.95
http://www.nolo.com/legal-encyclopedia/free-books/small-claims-book/chapter11-4.html