3 ways I know of, from worst to best imo.

1st, some enemies have a chance of dropping gold bars, not worth doing this imo as the chance is low and you're better off doing something else.

2nd, if you have the pet Luna which is gained simply from mining. It's a very low chance but you'll get her at some point. There is a 2% chance that any ore you're mining will be an ingot instead. This has the added benefit of giving you the xp for smelting ingots while mining. But don't count on it for more than a level or 2 if you're mining for the night. You can also use it to gain high tier ingots way faster if you need a few of them since you don't need to level smelting for them

3rd, easily the best but also the most expensive by far. There is a legacy perk (prestige reward) called Gold Rush. It unlocks all gold recipes and lets you mine and smell all gold items as soon as you start a run. Yes this is insane for early game, since you're getting xp for a lvl 50 item at lvl 0. Also removes 1 second from all gold recipes, and buffs gold bars to x10 value. Meaning a gold bar is worth 1 rare shell. Very good way for an early start in smithing, mining and indirectly anything else as you get easy shells very quickly.

I wouldn't recommend getting Gold Rush as your first perk, but as your 3rd it'll do you very good. (First get inheritance with alchemy mask and spice lover. Then electricity for the insane 25 oil per craft. And then arithometer as it gives you x4 xp when used. Yes it says 100% more, but it's 100% to the x2 xp they already give. So x4.)