Copper Bars are used for a whole range of different furniture, tools, blocks, weapons, and armor in Portal Knights, and most people tend to begin looking for these when they want to create Copper Pickaxes, Mini Drills, or the Arcane Compass.
It's actually pretty easy to get Copper Bars in Portal Knights, and you will need to craft this using a Furnace (a level 1 Furnace is fine). The crafting recipe is 3x Copper Ore + 1x Coal per Copper Bar.
If you need Copper Ore or Coal, head to an island to begin mining for these. Both are found abundantly on lots of different islands, and tend to be tied to be mutually exclusive. This means that islands that contain Copper Ore won't contain Coal, and vice versa. However, you should have no problem finding these if you have done some mining on a few different islands in the first world.
Once you have the material, craft them like you would craft any other item, using the Furnace instead of any other crafting table. Each craft takes 5 seconds, so it can take a while to create them if you need hundreds of Bars.