@planch -- When I was making cotton batiste curtains I used a really nice pima cotton batiste.
You can usually find them in sewing stores, and occasionally in fabric outlets, if you're lucky.
I assume you want it to be a kind of lighter fabric, since it's for a nightgown, so I would say choose the highest quality one you can find. The attendants in the shop should be able to help you out too, most of them are very experienced in matching fabrics to projects.
Best of luck!