Maybe an XS adult one at Target will fit her? Not sure of the material, but the nightgowns can be so soft. I'm assuming that you don't like the feel of those stiff nightgowns with the characters on them?
DD (6) wants a nightgown. Everything I find is polyester. Does anyone have a source for normal cotton nightgowns?? I'm about to put her into one of my old tshirts, but it definitely will not meet her standards!!