Halloween costumes

Where can I find a Halloween costume for my 4-year-old daughter? I know I've left it late, but I really wasn't planning on celebrating Halloween (even though I'm American myself, I'm very much against "exporting" holidays and customs that have nothing to do with other countries' history), but now she's been invited to several parties and I don't want her to miss out. Anyway, the Carrefour near me in Drogenbos doesn't have anything that would fit her. Any ideas? Other shops, preferably in Elsene/Ixelles or Ukkel/Uccle, or easy DIY ideas? Otherwise I'm going to have to improvise a ghost costume out of a sheet. Thanks!