I haven't read the thread Chippy refers to for a while, but the last place I managed to get white grape juice from, and this was a few years ago now, was Acado.
Having said that, I have just remembered that there is a Polish shop not too far from me that stocks it. I haven't bought any though, as I'm not really a wine man (more of a whine man). I'll be heading there again in a week or two to stock up on some other things so I'll have a look if they still do it. Not that it will help you, I guess, but might be a tip for others of they have Polish or other similar shops near to them.