You can buy gold bars directly from a shop or a merchant in dargonia when you gain access there, depends on your money, i don't remember exactly which town it is, but i think it was rakkake, i remember buying few gold bars there, after saving up enough money, just advance the main story to the point where you open borders with dargonia, their merchents have a variety of stuff you usually grind for, like gold, ice harpy feather, silk and more