Hello Barry,
Stone Plaster/Herculite is widely available in the UK, (maragon.co.uk, or I frequently just pick it up at my local art supplies shop) but what was it you had in mind to use it for? Personally I think it's a good solution to making terrain or other comparatively large things quickly & cheaply, but, like all plaster products it doesn't have the sort of strength to cast thin freestanding stuff, hope that helps.