Since Bismuth is diamagnetic, its magnetic susceptibility is negative (-13x10^6 according to Bozorth), so it will have a magnetic permeability (in Gauss-Oersted unit system) less than 1.
I am not aware of anyone measuring a BH curve for the material, but you can probably get a reasonable approximation by using a straight line that goes through the origin and has a slope of .999987
What is the maximum field that you'll be modeling? The method I described is basically extrapolating, and I wouldn't want to extend it too far.