I figured. It said it could be done from any of those kinds of missions, I imagine those are just the easiest places to do it? I really have no idea, still new but at least the Wiki gives one good spot
the wiki is definitely wrong there are other places you can get the parts.
i got the helm on wave 5 xini
This. You can do the very first raid mission on mercury and have a decent chance of dropping the chassis. I went through it 10 times and got the chassis at least 4 times. I'm not sure on the actual drop rate because anecdotal evidence is anecdotal.