We don't tend to use them here, but I did get DD some this year from target. They're an 8 (she's 5), and she still wears them occasionally. I got them where the boys undies and socks are ( but there's a girl section in that spot too).
DS is in 4's to get over his big noggin, but he refuses to wear them.....oh and books on the head don't work....wish I could find something that did - she's costing me a fortune in size upgrades every 5 minutes lol.