  Starspots, plages, and activity cycles cause radial velocity variations that
can either mimic planets or hide their existence. To verify the authenticity of
newly discovered planets, observers may search for periodicity in spectroscopic
activity indices such as Ca H & K and H$\alpha$, then mask out any Doppler
signals that match the activity period or its harmonics. However, not every
spectrograph includes Ca H & K, and redder activity indicators are needed for
planet searches around low-mass stars. Here we show how new activity indicators
can be identified by correlating spectral line depths with a well-known
activity index. We apply our correlation methods to archival HARPS spectra of
$\epsilon$ Eri and $\alpha$ Cen B and use the results from both stars to
generate a master list of activity-sensitive lines whose core fluxes are
periodic at the star's rotation period. Our newly discovered activity
indicators can in turn be used as benchmarks to extend the list of known
activity-sensitive lines toward the infrared or UV. With recent improvements in
spectrograph illumination stabilization, wavelength calibration, and telluric
correction, stellar activity is now the biggest noise source in planet
searches. Our suite of $> 40$ activity-sensitive lines is a first step toward
allowing planet hunters to access all the information about spots, plages, and
activity cycles contained in each spectrum.
