One source for these is Westfalia mail order - see: http://www2.westfalia.net/suche/?q=ferrules. Alternately they should be available from the usual electrical suppliers and ebay. It is, of course, essential to use an appropriate crimper: fortunately these are not expensive and are available from the same sources.

Regards,

Yes, Westfalia is a source, but is nowhere near as reasonably priced as crimpterminals.co.uk; I've searched ebay for ferrules on a few occasions, but again never quite found a bargain.