Don't buy - instead, ask friends, families, or even neighbors if you can borrow! You may also want to consider a Kids Clothing Swap between yourself and friends and others, to save money year round and not just on formal attire :)
I organize my child's drawers with all the latest sizes that she wears, current season only. Everything else that doesn't fit or is too big (or a diff. season) goes on the shelf in the closet (to get ready to donate or for when she outgrows the current ones).