Big union jack flag

Anyone know where I could buy a BIG Union Jack flag? Can't be mail order, as I need it for Saturday. I've been to a shop on the corner of rue du Lombard and rue de l'Etuve (between Grand Place and Mannekin Pis) in the past, but I asked a friend the other day who was in the area for lunch to look and he said that there was no flag shop anymore.