I touched briefly on this nightie in my earlier post about the Magee of Donegal robe she wears over it - just to say it hadn't been identified.
Unfortunately we will probably never get a firm ID for this. The good news, however, is that it's not a particularly unique nightie. It's cotton and white, has short, wide sleeves, hangs loose to her knees, and has a pretty eyelet pattern across the chest. I actually have a bit of a thing for big, loose old-lady nighties so I can safely say it's really easy to find a decent alt for this piece. Just walk down the nana aisle and you should find something.
Amy wears the same nightie in both The Beast Below, on the morning of her wedding in The Big Bang, and in the Night and the Doctor minisodes Good Night/Bad Night.