Go to the RHS website and select Plant Finder.
It only lists two suppliers, but they both offer mail order

Try contacting Clematis growers directly such as this company who list it - The Walled garden Clematis nursery Brinkworth House, Brinkworth Malmesbury Wilts SN15 5DF Tel: 07921 436863 or e-mail fraser@clematis-nursery.co.uk

Thorncroft Clematis Nursery in Norfolk used to list this clematis but had a crop failure a couple of years ago.
Although they don't list it at present, it might be worth giving them a call.