  We present a new field theoretic approach for finite dimensional site
disordered spin systems by introducing the notion of grand canonical disorder,
where the number of spins in the system is random but quenched. We analyze this
field theory using the variational replica formalism. For a variety of
interactions we find a spin glass phase (with continuous replica symmetry
breaking) and explicitly discuss a three dimensional system where this occurs.
This approximation also suggests that any ferromagnetic transition occur within
the spin glass phase.
