The emissivity is the most representative radiative property and it has to be entered in the boundary condition panel for heat transfer. At inlets and outlets the default emissivity/absorptivity is 1 (all the radiation exiting the domain does not return). From experiments, the non-metals have an emissivity between 0.8 and 1 for most polar angles. Metals emissivity is between 0.04 and 0.06 increasing rapidly for polar angles larger than 50 degrees with the normal.
You have to distinguish between the internal and external emissivity. The internal emissivity is used to compute the radiative exchange inside the computational domain whereas external emissivity refers to radiation exchange with the exterior. Ideally, electromagnetic wave theory can be used to predict all radiative properties. But this theory neglects the effects of surface conditions. Consequently most of the data available today is from experiments in the 1950s and 1960s. Any radiation book has a number of references on such properties.