also try Tesco (in their car accessories aisle etc) and Asda. I picked up a pack of one poncho and one tin-foil-type blanket from Tesco for less than £1 last time I bought them there.
Will you use ponchos though, in November? I would have thought that your winter coats and hats would keep you dry? The only time we've used ponchos at DLP was in May/June when it was warm but torrential rain.