The real part of the complex relative permittivity is n^2-k^2 as you have said, and the imaginary part of the complex relative permittivity is equal to 2nk. The loss tangent is then simply the ratio of the imaginary to real part of the complex relative permittivity. In terms of a reliable source of dielectric data, what sort of materials are you interested in? Biological (ie tissue) or inorganic materials, or something else?