There certainly is here in Australia, a number of them. One with a great reputation is Classic Industries Australia who make them for virtually every brand and model motorcycle that is likely to want to tow a trailer. Hitches and Trailers. There range is enormous and they sell international via online orders.
Classic Industries :: Features of Hitches and Towbars for motorcyles, scooters and motorbikes
Classic Industries :: Motorcycle Trailer Hitch and Towbar :: Shop
TT17 Triumph Rocket III 2003 to 2009 Au
$475.00
TT24 Triumph Rocket III 2003 to 2009 with Jardine exhausts and Roadster 2010 onwards Au
$475.00
TT21 Triumph Rocket III Touring 2004 onward Au
$475.00
They make 3 versions for the Rocket 3s an early 3 pipe model one and a later Roadster one as well as a Touring version.
@Mittzy has a second hand one for sale in Brisbane Queensland Australia at the moment.
Towbar (Classic Industries TT24)- Roadster. AUD $300 + Shipping
For Sale - Mittzy's Clean Out Time
Link Removed