I'd suggest looking on dedicated manga apps such as Manga Plus or Crunchyroll Manga. They often have a wide range of titles available, including Chained Soldier.
It's hard to pinpoint exactly. The ending point can vary depending on how the anime adaptation was structured compared to the manga's original storyline.
Well, without having read it specifically, I can only guess. It could be about a military - themed story where the soldiers are connected in some way, like in a chain of command or a more metaphorical sense. There might be a plot that revolves around loyalty, sacrifice, and the struggle for power within the military structure. It could also introduce some supernatural or fantasy elements that are common in light novels to make the story more exciting and engaging.
Well, sometimes it's available on cable TV channels that show classic movies. Or you could look on Hulu. But availability might vary depending on your region.