Where can I get an authentic replica of a Barrister's wig?

I don't know. Am only answering as you've had no answers yet, but if it was me I'd try ebay (although it might be difficult to know if it's genuine i suppose) or maybe google barrister wig and see what happens. Maybe google barrister uk and if it gives any law firms you could email them and ask where they get their wigs from. There must be a company somewhere that makes them.