Yep, a muscle suit is designed to go under clothing and gives the wearer a muscular appearance. But that makes it easier knowing you just want the under armour.
The most common form people use is a black body suit - simple, easy enough to find and somewhat cheap. Works pretty well in most circumstances. But as you're looking for accurate, there's at least a couple more ways to go. The first would be to make your own. Here's a couple of threads I remember where members have made their own, by combining foamsmithing and sewing skills:
Hey everyone! This will be where I post progress and details of my new Reach suit. I've created two Reach suits in the past, one for myself and one for a friend. Hopefully this one will be even better looking and better functioning. This suit will be pretty much all foam, made with pep patterns...
www.405th.com
So I started doing an unfold for the Mk 7 undersuit and intended for it to be a prototype… never intended to start a new build but you guys know how that always turns out. I am using TNT 4mm foam for these pieces. I built this with flexibility in mind. Each panel has nylon webbing between...
www.405th.com
Secondly, you could look for a body suit with the patterns you're looking for on it, such as a Master Chief costume you can get in stores (though I see you too are an Aussie, and I don't recall seeing any chief suits in retail)