I've always heard the little wire "commando" saws were complete and utter crap - they'd break in moments of use and weren't worth the trouble. So I've never messed with them, despite an interest/mild obsession with small survival/contingency kits. This video - an old one from IAWoodsman that I just stumbled on - shows the proper use of one. He makes a perfectly useable bow saw without too much trouble.
Note that a saw like this wouldn't really be particularly needed for processing firewood - just chuck what you can find on the fire. It would be useful in shelter building, trap building and other bushcrafting. Your mileage will vary on whether that's important to you.
Interesting to know, though, that, at least with the right saw and correct employment, these aren't entirely useless.
BCB, I believe, makes the military issue ones. Like most things, they are available on Amazon.