The default source for the Army firesteels used to this Bagheera fellow on bladeforums and knifeforums, but he can no longer provide them due to some new contract his provider entered into. It's a shame too because he used those sales to fund his scout troop.
If you're in Europe, just google for 'army fire steel' or 'swedish fire steel' and you'll find a bunch of online retailers. If you're in the US, you can get the Army (big one) from
http://www.survivalsheath.com and from Emerson Knives. If you want the scout model, they're re-branded by Kershaw and they're all over eBay.
I haven't yet used my Army in the field. I don't really think the fact that it's bigger will guarantee a greater degree of success - I think it just lasts longer - which is relative anyway considering that even a small sparker should lasy years.
Truth be told, I prefer the Doan magnesium one over all the others. The sparker rod is of high quality and seems pretty hot. Though I don't usually use the magnesium part it's nice to have if your tinder is less than prime. Most importantly, I find the rod-only devices too hard to hold firmly by just the thumb-piece.