I wasn't actually thinking halloween until I saw this awesome SALE at BuyCostumes.com
Take an additional 50% off all SALE items!
I have seen some great deals and recommend you hurry so you can get what you want.
Another thing you will want to watch for is 5 deals under $5 every day throughout the week. The last day is Friday! Here you will find some great costumes and party supplies under $5 (Everything today was at least 85% off). I am not sure if you can use the extra 50% code off though. Let me know if you try.