Try Spurgeons, who are based in Crawley, Sussex.
They supply ones that they get in from the States, they have been the best ones that i have come across, as they are fairly robust and always snap off nice and clean, so no little bits of polystyrene end up floating around. You may need to buy a whole box, which is quite a lot, but this should last a good while, especially if they are only used for larger items of furniture.
Regards
Mark