When the battle is over, there is going to be some equipment left on the field, which belonged to men who are now dead or who dropped it when fleeing. Much of it is likely to be broken, but one might be surprised at what's still useable: swords, boots, saddles, even just buckles and leather straps. The larger the battle, the more likely that there will be stuff around that your men can salvage.
Naturally, not just anyone can waltz onto a field of battle. The victors are going to jealously guard the field -- that was, after all, the point of fighting over it -- and any unit which was scattered or retreated will be unable to forage the battlefield. This includes units who were on the winning side of the battle.