Copper Bars are very obscure items when compared to other items, having the inability to be dropped by any enemy from any area or as loot from chests. Therefore, their only source is from trading with Merchants and relationship gifts, with the shops of Pompon and Shello Bay often stocking up on Copper Bars, while Asteria can provide Copper Bars alongside the other three basic bars.
Within trades that grant Copper Bars, Coins, Copper Ore or both are used.
Copper Bars are resident to a small subcategory of items: trade items. That being their only purpose within Postknight is to be used to trade for different items to aid the player in their adventure. Their most prominent trade in this regard is unlocking the first and second tiers of Armor and Weapon sets, enabling a newer player to progress through the game with better equipment they had not collected by campaign completion.
Aside from unlocking equipment, Copper Bars are very useful in trading up for the bar of the next tier, the Silver Bar. Given how trades of Silver Bars can be a limiting factor, having the option to trade up Copper Bars once their related equipment is fully unlocked is a viable alternative.
Once unlocking of armor is complete however, Copper Bars have very little to no value to a late-game player, since the trades possible with it only serve a single end goal.