There are multiple literature references from which you can get estimates for the conductivity, e.g.

• S. Gabriel, R. W. Lau, and C. Gabriel, “The dielectric properties of biological tissues: II. Measurements in the frequency range 10 Hz to 20 GHz”, Phys Med Biol, vol. 41, pp. 2251-69, Nov 1996.
• L. Manola, B. H. Roelofsen, J. Holsheimer, E. Marani, and J. Geelen, “Modelling motor cortex stimulation for chronic pain control: electrical potential field, activating functions and responses of simple nerve fibre models,” Med Biol Eng Comput, vol. 43, pp. 335-43, May 2005.

In a three-shell EEG volume conduction model (either spherical or realistic)the conductivity ratio's of 1, 1/80, 1 are commonly used for brain, skull and skin.

It is commonly assumed that the impedances of the tissues relevant for EEG and MEG source modeling are purely resistive, i.e. there are no capacitive or inductive effects. This means that the impedance is frequency independent. The attenuation of the source activity at the skull therefore does not depend on the temporal or frequency characteristics of the source.