The Alpine Garden Society's website has lots of information on other plants as well as alpines plus information on their shows etc.
Bluebell Nursery are a supplier of rare trees and shrubs.
The Flora of China is the reference work for many new plants coming into the country just now.
A good site containing numerous links to other gardening site etc. A source of valuable information.
The Royal Botanic Gardens in Edinburgh is one of the very best gardens in the country at the moment.
The worlds largest gardening charity with numerous links and pages of information on all of their flower shows etc.