You should contact sales@atus.com. They'll be able to tell you what you need (I think it might be the AT8300 cable). If they won't sell directly to you, they'll probably suggest a retailer who can order from AT for you.
I did this for my AT863Rx when I could not find a set from any retailers. AT still had some in their warehouse and suggested I use Full Compass who placed the order on my behalf. The mics drop shipped to me from the AT warehouse.