Best Choose – Military Modular Sleep System
Military Modular Sleep System
You can get them new on Amazon or used on Craigslist pretty cheap.
These are FANTASTIC bags, if you're not worried about weight. They're good down to -30.
One thing you need to know; there are two version of these. The one that OP linked to is the "old" (4 piece) version, which is most obvious by the woodland camo pattern. I have this one and it's perfect for what you need if you can find one for a good price.
The new-er version is a 5-piece set in ACU camo, which supposedly is even better made than the old ones. Here's an article with pictures of both: http://www.tacticalintelligence.net/blog/military-sleep-system.htm
One caveat: Shop carefully, as there are fakes of these on the market.
been using mine for 30 years, didn't call it a system when I got it though.