Horseware Ireland has an adorable kids line!
Dover, Equistar, and Facebook groups!!
Facebook group Just for Ponies and kids has a lot of resources
Try tackhunter.com or the app. I’ve gotten some good stuff for pretty cheap there, and you can search by the size you’re looking for.
Dover Saddlery has sizes for even the tiniest riders. Shop online or you may live close to a retail store :)