You should make sure to visit Calmcacil in the city of Minas Tirith. He offers some of the better store-bought platemail. I think the best combination of orders is a high-steel plate cuirass. The armour is very expensive (~6000 copper), but fortunately there is a bank nearby.
Visit Gunthor, the Master of Advice in the Third Circle(I think) of Minas Tirith. He has some tasks for you.