>>213719693
Surplus USMC goretex bivvy, sometimes you can find them on their own for sale on eBay.

Snugpak makes a good bivvy too, you can buy each piece individually if you don't want all the different layers and sleeping bags.

Personally I think you're better off with a compact 1 person tent, NorthFace and MSR make good ones that are about the same price as a Goretex bivvy.