  For the joint analysis of second-order weak lensing and galaxy clustering
statistics, so-called $3{\times}2$ analyses, the selection and characterization
of optimal galaxy samples is a major area of research. One promising choice is
to use the same galaxy sample as lenses and sources, which reduces the
systematics parameter space that describes uncertainties related to galaxy
samples. Such a "lens-equal-source" analysis significantly improves
self-calibration of photo-z systematics leading to improved cosmological
constraints. With the aim to enable a lens-equal-source analysis on small
scales we investigate the halo-galaxy connection of DES-Y3 source galaxies. We
develop a technique to construct mock source galaxy populations by matching
COSMOS/UltraVISTA photometry onto UniverseMachine galaxies. These mocks predict
a source halo occupation distribution (HOD) that exhibits significant redshift
evolution, non-trivial central incompleteness and galaxy assembly bias. We
produce multiple realizations of mock source galaxies drawn from the
UniverseMachine posterior with added uncertainties in measured DES photometry
and galaxy shapes. We fit a modified HOD formalism to these realizations to
produce priors on the galaxy-halo connection for cosmological analyses. We
additionally train an emulator that predicts this HOD to $\sim2\%$ accuracy
from redshift $z = 0.1 - 1.3$ that models the dependence of this HOD on 1)
observational uncertainties in galaxy size and photometry, and 2) uncertainties
in the UniverseMachine predictions.
