I got my binder from gc2b and it's wonderful. They're of nice quality, come in a variety of nude colours (dark and light!) and, dare I say it, are actually comfortable for a binder. The site has a little info section about their sizing and how to measure, so it's easy to figure out the size. Sizing ranges from XXS to 5XL, so they come in many sizes too. They're reasonably priced too, and are honestly more than worth it.
Definitely recommend their binders, so check them out for your child!