Where do I obtain Legionnaire Armor without killing them?

You can usually get some when doing the Battle for Kvatch, early in the Main Quest. There are three Legionnaires who tend to get themselves killed.

The battle to retake Kvatch often results in several legion casualties whose gear you can loot.

Once will o wisps start spawning, you can travel the road to Leyawiin where the wisps spawn and wait for a guard to get into a fight with one. The guards are not usually going to survive an encounter with a wisp.