I'm sorry. I've just checked, and I think there is actually no muscle cuirass to be captured/bought, in neither plotline (apart from killing the general in the Roman camp as discussed above; but of course this isn't really feasible, though you might perhaps try to murder him at night, or to get him to drink firsta few regular cups of wine and then a poisoned one).
As far as I remember, I'd originally equipped the Centurion who holds you up with his men (while you're on your way to get the Usipians' treasure) with such a cuirass, but later decided against this for reasons of historical authenticity.
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Basically, as far as I know, nobody wore this type of cuirass except for the emperor and his generals. It was no good for active combat due to it enormous weight and rigidity. Instead, it was meant to look representative (see the picture of Augustus in the Teudogar intro; he's wearing this type of cuirass; of course his is magnificiantly decorated, with a propaganda scene showing the Persians asking for peace). Its only practical use was to protect the highest-ranking officials from assassination and enemy missiles while they directed combat operations, standing far away from the line of battle.
The troops who actually did the fighting wore chain mails shirts, or the newly developed segmented cuirass (lorica segmentata), which was both very light and flexible, and offered great protection because the metal was solid enough not to be penetrated, and yet was designed to crumple when hit, like the crushing bin of a modern car, which absorbed most of the energy of every hit. So this segmented cuirass is definitely by far the best available type of armor for active combat. (Chain mails are almost equally good, but weigh twice as much.)
Lower ranking officials like a Centurion (commanding 100 men) usually didn't participate in active combat either (the only kind of combat they'd usually do was to punish their subordinates by beating them with a large wooden stick which they carried explicitly for this purpose). These people usually wore scale mail shirts, which looked pretty impressive but didn't offer much protection.
So in total, in 12 B.C., when there were about 30,000 to 50,000 Roman troops beginning the invasion of Germania, there were probably several thousand segmented cuirasses plus several thousand chain mail shirts (regular infantry legionaries), tens of thousands of leather armor (axiliary troops), several hundred scale mail shirts (lower ranking officers), and probably only a few dozen muscle cuirasses (the generals). This makes your capturing this kind of armor about as unlikely as Iraqi insurgents capturing Donald Rumsfield or General Sanchez...
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But I agree that for reasons of gameplay I ought to provide at least one of these cuirasses for you to capture. (Barter doesn't make sense since anyone wearing such a cuirass would be rich and not in need of anything, except for the solid protection offered by his cuirass.) I'll equip the leader of the Roman troop with the hostages with a muscle cuirass. This'll be available when I release the next minor update (which I'll do when the Italian translation is finally complete; which it should be sometime in the near future). Sorry for not thinking of this before.
