A quick search under goggle images for "bothy bags" will turn up a pot full of them
But here in the US you would be hard pressed to come up with a proper bivy sack. I am not talking sleeping bag covers here, but real climbing bivy sacks. The kind that will make the difference between survival and death if caught out in really bad weather in the mountains.
Thankfully I know one place that can square that situation away, Brooks-Range.
Half the size of a Nalgene 1 liter bottle and 8 1/2 oz on my scale for the two man version.
Not bad in a pissing cold rain either.