  We consider the stability conditions for half-quantum vortices in quasi two
dimensional $p_x+ip_y$ superconductor (such as ${\rm Sr_2 RuO_4}$ is believed
to be) through energetics calculations. Although the predicted exotic nature of
these excitations have recently attracted much attention but they have not been
observed yet. We pay particular attention to the fact that an isolated half
quantum vortex has a divergent energy cost in the bulk due to the associated
{\it unscreened} spin current, which requires two half-quantum vortices with
opposite spin winding to pair. We show that the stability of such a pair in
bulk is enhanced when the ratio of spin superfluid density to superfluid
density $\rho_{{\rm sp}}/\rho_{\rm s}$ is small. As a step towards stabilizing
and observing these exotic excitations, we propose using various mesoscopic
geometries. We find that certain mesoscopic geometries can selectively allow
only single half-quantum vortices to enter the sample for small $\rho_{{\rm
sp}}/\rho_{\rm s}$.
