May I know which country are you from? We are actually from Singapore. We were refered by our hospital here to contact their local suppliers directly for my son's size. Alternately, you can check with your local pharamcy (which usually carry only adult sized stockings) and ask them to indent for you.
You may also consider ordering online (eg Amazon.com) as there are many selections available. Sizing charts are available for each brand (you will have to individually search the manufacturer's website for the sizing charts).
Also you may want to first discuss with your daughter's doctor on the suitability of compression stockings (and the required grade of compression).