I was in Sainsburys today and in the womens' section (it wasn't weird that I was hanging around there, since I am a woman lol) they had ... well, basically, onesies for women. I just stared at them and thought, WTF? I can't remember much about them, they were basically just T-shirts but with a crotch part like a onesie (though the crotch didn't have snaps or anything).
I don't know what material they were made out of, but they looked stretchy.
Might be worth a look next time you're in Sainsburys, I will have a closer look next time although I don't know if they will have them in my local one. The place I was at today had a pretty big clothing section.