Golden caster sugar is unrefined caster sugar. Since caster sugar is finer than than U.S. sugar,
you can make your own. Grind regular white granulated sugar in the processor until it's super fine, NOT powdered.
The ideal substitute, however, would be raw sugar, such as demerara or turbinado, processed until super fine, but I can't find any info on whether or not that can be done successfully.