There are plenty of CoD guys with fatigue pants and caps. Newer CoD figures have removable headgear, so you can swap the cap onto a different noggin with sunglasses or a beard or whatever you're after. And there exists a CoD torso with a buttonless shirt. These things come in every military color. You're likely looking at mixing and matching parts from a few figures to get the exact combination you're looking for.
That's how it starts.... one lazy-bastard custom, and now I probably have 700 of the little buggers.
I'd check Dollar General, as they seem to still carry the multi-packs, and you might be able to spot the useful parts you need. Just note, CoD/Destiny and Halo/MOTU use slightly different construction, so the arms, waist, and torsos are not 100% compatible. The heads and legs are pretty universal. You should try to stick to one flavor for most of the parts, and I would suggest CoD for any real-world-ish military customs, as your part selection will be substantially greater.
Another good source is ebay, just browsing the multitudes of loose figures, and Aliexpress has a lot of KO figures -
but they are often not 100% compatible with official stuff, so I try to keep the KOs mixed and matched to themselves.
And of course, if you can't find everything in the right colors, these things are easy to paint. Although I always strive to find arms and legs in the right colors to minimize the amount of joints that need painting, as they're prone to rubbing.