If you want to make your own Halloween costumes, or just dress up clothes for your kids, A. C. Moore is the first place I would shop for all the supplies. If you go to here http://www.acmoore.com/Projects/holidayproject.asp?ProjectSubCategory_ID=14&ID=280
there are two costumes with a supply list and instructions.
You can make dress up clothes for your little ones with supplies found at A. C. Moore such as tulle, felt, elastic, fabric remnants, and items to adorn old clothing you already have and by adding a few little items you can turn it into part of a costume. Just look at a picture of something like a fairy, go to A. C. Moore and pick up some tulle, sequins, ribbon and anything else that comes to mind to turn an old slip or dress into a fairy gown.
The sales clerks and craft instructors are very helpful and if you tell them your ideas they can point you in the right direction and you'll be on your way to some great costumes.