Ness Botanical Gardens holds a number of gardening courses and horticultural classes. There are a series of gardening lectures and workshops you can look at too. If you are able to attend Reaseheath College at Ness Gardens - it has a variety of horticultural type courses targeted at professionals
They also have a good plant collection at Ness - one of my favourites is the primula vialli, which were collected by George Forrest on the Chungtien Plateau, Yunnan in 1906, as Primula littoniana.
If however you're interested in learning remotely, or distance learning gardening courses, or even just learning more about garden design or horticulture for fun - please come and drop by at MyGardenSchool.