There is a nice Rigby 577/500 BPE Magnum at Vintage Doubles and Champlin's has an Alex Henry.
There is a W.W. Greener on Gunsinternational which is attractively priced too. All Jones underlever hammer guns.

I have heared that the 577/500 is the best of the BPE .500s too. I imagine it would be from the looks of it.