Where do I get the Legionnaire Imperial armor set to solve "Missing in Action"?
You can't. The linked answer is incorrect; such armor does not exist.
It's actually not possible to complete this quest peacefully. There is an Imperial Decree that is not available without the use of the console that appears to be the key to this quest, but it was apparently excluded from the game as the result of a bug or some other unknown reason. Not only is it not present in the actual game, but the console-obtained version doesn't do anything.
Source: UESP Quest Page
The strategy guide says that there are 3 options.
- You kill everyone and rescue Thorald.
- You bring Avulstein Gray-Mane with you and kill everyone and rescue Thorald.
- You stealthily sneak into the Keep through the north side door that is a master lock.
Edited. UESP has conflicting information, attempted and failed. The option isn't available, but should be. Some programming error or something keeps it from working right. So, no not until it's patched or fixed either by Bethesda or modders. No peaceful solution exists, and there is no such armor. A book related to the quest can be gained using console commands, but it does nothing aside from take up inventory space.