Well I got some form Technology Supplies. But I don't think they are selling anything at the moment. You can get it at B&Q although it is expensive. Other than that I suggest you check out local glaziers. Depending on how big a peice you want they may be able to give you an offcut that would do.
That's the place I've used before, nice and quick with delivery, but I don't know how good the prices are as it's the only place I've looked -Sam
yeah i used them to, nice and fast delivery, but a bit more expensive then this place. http://www.technologysupplies.co.uk/acatalog/Acrylic_Sheet__Rod__Tube_and_Block.html I dindt use them because you have to ring them up and place a manual order, i should have done and could have saved £7.