Where is the best place to buy 500 Nitro Express and 470 Nitro Express brass? Thanks.

Buffalo Arms may still have some old BeLL.

Jamison from one of the online retailers (Grafs, Midway, etc...) if you can find it in stock.

Or sometime this fall, both should be readily available (and cheaper probably) from Hornady.

You may also try David Little @ Kynamco in London or Simon Clode at Westley Richards in Birmingham.