Check our forum thread for updates on the best deals around on halloween costumes.  Thanks for your hard work on this list Abby1.

Dollarama
-Costume accessories $1.25 (while supplies last)

Chapters
-20% off select kids Halloween costumes online only (while supplies last)

Gymboree
-60% off selected Halloween costumes in-store and online (while supplies last)

Party City
*limit one coupon code per online order*
*Halloween costumes can only be refunded until Oct. 17, otherwise it is exchange only*
-20% off your entire Halloween purchase in-store (one per family) or online (use code CALR96) (Ends Oct 16)
-20% off your entire Halloween purchase in-store (one per family)or online (use code CAW28P) (Ends Oct 16)

Spirit Halloween
-20% off a single item in-store or online (use code LOCATE12) (Ends Oct 31)
-20% off a single item in-store or online (use code Signup) (Ends Oct 31)
-20% off one single item online (use code SPOOKY) (End date unknown)

Zellers
-30% off all Halloween costumes for the family (Ends Oct 18)