The Agroforestry Research Trust is a great resource for plants and information on forest gardening, based in the south west of England.
Plants for a Future is an online database with details on the cultivation and use of thousands of plant species.
The RHS Plant Finder helps you find the species you need, with details of both Scottish and UK nurseries.
Further information and discussion
There are various e-groups where you might get a useful answer to forest gardening questions. http://tech.groups.yahoo.com/group/ForestGardening/ is a dedicated forest gardening group but doesn’t have much activity at the moment. http://uk.groups.yahoo.com/group/scottishpermaculture/ is a more lively group which carries discussions of all things permaculture including forest gardening.