  The detection of resonance absorption lines against known objects such as
individual galaxies and clusters of galaxies is a powerful approach for
studying the gas content of these systems. We describe an efficient method of
identifying background quasars suitable for absorption line studies. In this
finding technique, we identify serendipitous X-ray sources, about 1/8 of which
are suitably bright quasars at moderate redshift. We identify 16 new quasars
and galaxies with active galactic nuclei (AGNs), and confirm 5 known quasars
and AGNs, superimposed behind elliptical galaxies and clusters of galaxies. We
also present 3 QSO/AGN candidates with uncertain redshift identifications.
