Share 'Where can I buy cheap card in London?'

Hi. I have all my pages ready for the zine sympossium (which I can't wait for), but I am need some cheap coloured card for the cover. Does anyone know where I can get some? I am doing it A5 size and I will be photocopying onto it so it can't be too thick. I have heard A3 is cheaper but I don't have a guillotine so really I am looking for A4, if possible. Any suggestions would be very welcome. Th…