B.C.
When my wife and I were shopping for good deals on "used" kids clothes, we would go to garage sales or church "garage" sales.
You will find the best sales the weekend before the first of the month. Why? Because most people spend all of their money before the end of the month so their is less competition.
You probably have several friends that are addicted to garage sales. Ask them to call you when they find a good deal on the size of clothes you are looking for.
You probably already know this, but I like to go to garage sales in the rich neighborhoods. They have the money to buy the very best and the ability to only wear the clothes a few times before the get rid of them because they are tired of them or they are "out of style". Additionally, rich people think a lot less of "used" clothes and will price then real cheap to get rid of them.
Good luck to you and yours.