You can get them from smelting copper ore in the mining profession! 5 ore collected equals one bar smelted, if I remember correctly. The first level of the mines should drop a ton of it!
I can give you the two you need to start out with if you'd like, too, I have a bunch of extra ones.
Cyber has it right, but I've also listed just a few cheap copper bar listings on the FM in case you're interested (or if anyone else is having the same issue!). ๐