I was going to direct you to Peter Kummerfeldt's site but UncleGoo beat me to it. Keep in mind that the the bags Peter sells are great but they weigh about half a pound each. If you are shaving weight you might be better off with the AMK HeatSheets emergency bivy sack at 3.5 ounces. It is not as thick & bombproof as a 4-mil bag but it is much tougher, quieter and more repairable than the old school space blanket material.

I think it is a fair compromise for the weight (and space) conscious. Ounces turn into pounds, which turn into pain.