Those violas are mainly bedding violas (most suitable for garden use), rather than show or exhibition types.
A couple of references on violas, which concentrate on the bedding types:
- Morris May: 'Victorian Visions' in 'The Garden', May 1998, p.356-361
- Mike Hardman: 'Viva Violas!' in 'The Garden', May 2008, pp.298–301
As regards supply: Wildegoose Nursery,
http://www.boutsviolas.co.uk, who will ship to the EU as well as Britain. (Mark and Stephanie Roberts developed the quite well-known Bouts Cottage Nurseries over the years since 1978. When they retired last year, the nursery was taken on by Laura Crowe and Jack Willgoss, and hence the plants can now be found at Wildegoose Nursery.) This is one of the best collections of violas for sale.
An important constituent of bedding violas is Viola cornuta, from the Pyrenees (as John hints). There are many lovely cultivars of that species, which can be grown in similar situations to bedding violas. Not as easy to get hold of, granted, you can read about V. cornuta in my article in 'The Plantsman', March 2007. Cornuta genes add perenniality, and can be seen in some cultivars by the presence of a white eye in the middle of the flower (many other violas and species 'pansies' have a yellow eye).
A lot of the development of V. cornuta was done by Richard Cawthorne, in Kent. Morris May, in his 'Planta Vera' nursery between Woking and Staines, took on Richard nursery contents shortly before Richard's death, and for a while he was the national collection holder. But he had to focus on more commercial plants (mass plantings for councils, etc.). If Morris has any violas left, you'd have to go visit and ask nicely! (It would help to mention my name, and send Morris my regards.)
The best place for V. cornuta cultivars (they sell other violas and other perennials as well) is Elizabeth MacGregor Nursery at Kirkcudbright, in the west of Scotland -
http://www.elizabethmacgregornursery.co.uk. They ship to the EU. They are regular RHS Chelsea medal winners. A photo
here shows how V. cornuta can form lovely spreading carpets. You may wonder about the suitability for mediterranean climates of plants that grow well in the west of Scotland, but as proof, one can point to the superb cedars of Lebanon that grow in their garden. The web site and catalogue is quite nice to browse, helped by Alasdair MacGregor's excellent photography. Roy Lancaster visited their nursery and garden, as
documented in 'The Garden', May 2008.
As I have made significant progress developing the structure of my garden in Cyprus, I am almost ready to try several V. cornuta cultivars myself. I am looking forward to that.