Japanese Garden Society - the name says it all: open to anyone interested in Japanese gardens.
National Gardens Scheme - gardens large and small that open to the public for charity. This link will take you directly to the part of the NGS website listing gardens that feature Japanese plants and/or landscaping.
Japan Society - the society has been at the centre of cultural links between Japan and the UK ever since it was founded in 1891. During Japan 2001, the year-long festival of Japanese culture, it was the main coordinator for all garden-related activities, including a fascinating exhibition about the way Japanese plants came to Europe. The society's website used to maintain an archive of materials related to the exhibition and other garden-related activities during Japan 2001. Sadly, this is no longer online.
Royal Horticultural Society - if it's gardens, plants or anything related to them you want to know about, this is the place to start looking.
Kew Gardens - why pick out Kew when there are so many other wonderful gardens in Britain? Not only because the gardens are beautiful, but because Kew is first and foremost a scientific institution - a veritable encyclopaedia of knowledge about the plant kingdom. What's more, it has a wonderful collection of Japanese plants and landscape gardening.