Unless I'm mis-remebering things I believe that they're exotic paper-in-oil film caps, not electrolytics. I think the first version used some soy bean oil cap or something like that.
As far as I know there aren't really any more 500V blackgates. In this case I think we're talking other caps, he used some exotica on the first 10 and had to use something else on the next 10. I wonder what he's using moving forward?
Do you want to build or buy? I'm guessing buy, given the price range. If so I'd find your way down to Florida and see what catches your eye. I think there'll be at least a couple new drool worthy options to consider.