The kids and I went to Shimojima in Asakusabashi to see what they had for Halloween decorations. Shimojima is primarily a paper goods store. It has floors of packaging....boxes, gift bags in paper and plastic from extra small to humungous. They have a floor of stationery, a floor of cleaning products, washi paper, stickers and receipt pads. It is, I assume, where businesses go to shop to buy a lot of their supplies. As part of their service of supplying businesses, they also offer seasonal decorations. Many businesses hang things from the ceiling for each season (Valentines, cherry blossoms, summer. fall, Halloween, winter, Christmas).
They had a huge selection for Halloween...it filled the first floor. They even had the huge inflatables that are so popular in the US. We bought one package of spider webs, a small plastic cauldron and a cute plaque to hang on our door (all for about $11). The price is right and the selection was pretty good.