Yep, I love OUAC. Purple Cow is a resale on Gravois, but they don't have quite the selection and I think the prices were higher (I was only there once). I've also gotten a few things from Goodwill. OUAC is by far my favorite resale for kids/maternity clothes.
I've also had good luck with Purple Cow and Once Upon... The Goodwill in South County (kitty corner to Ronnies) has at least one rack. Value Village has SOME stuff, but you have to dig.
Re: Maternity Clothes