Saw them advertised once, a few years ago Ian, but far as I remember, they were from the US.
Personally, I think they only work in photographs (of models), and look rather silly otherwise on a static model. The only time you'll see a prop blur in real life, is in a 'still' photo, as the shutter speed of the camera 'freezes' the action - when viewed 'live' a spinning disc will be seen - if lighting conditions allow - or nothing at all.
In the few instances when I've built a model in a flying attitude, I've either added a clear, or slightly diffused disc, or omitted the prop blades and filled the holes in the spinner.