Sounds like you're describing a G.I. poncho and liner with an added zipper kit. I've got one and if I'm anticipating cooler weather, I include a fleece bag liner. While I can't give it an official temp. rating, it's comfy down to pretty d*#%@d chilly. The whole roll can be put together through Brigade Quartermaster, but if you've got a surplus shop close by, the poncho & liner will be cheaper there, and my fleece liner came from Walgreens. I don't know of another source for the zipper kit.
Have a great day!
Troy