In 1.3, they were a rare drop from common imps. Now they are much rarer in the world: Gold chests in the cavern layer, most of those don't even have cabins now, so you basically need a Spelunker potion to find them, and then its a 1 in 40 chance per chest. The flip side, however, is that you can grind them with lava fishing, which is involved but ultimately straightforward:
- Go to the Underworld. If you don't have a Golden Fishing Net, craft a Bug Net with hellstone to make a Lavaproof Bug Net. Get Hellbait, as much as you can. (If you got the Hotline Fishing "Hook" from the Angler, you can skip this, but that's a Hardmode award.)
- Make a lava lake at your base, or any other place thats safer than fishing in the Caverns, much less Underworld. You also need a Shadow Key to open the Obsidian Lockboxes.
- Max out your fishing power in usual fashion (equipment, potions, moon, weather, time, luck). Especially get Crate potions, brew your own if you have to. Don't forget the Garden Gnome and torches -- a Bone torch will help some in Forest, but you can get a bit of extra luck by putting your lava lake someplace where biome torches work.
- Use the Hellbait to fish for Obsidian/Hellstone crates. You're looking for the Lavaproof Fishing Hook, an accessory that lets you fish in lava with any rod/bait. It's almost a 1 in 5 chance per crate, so you should get one fairly quickly.
- You might have been lucky and gotten a Lava Charm already, if not, equip the hook and switch to Master Bait to fish lots of lava crates, until you get the Lava Charm. It's still a 1 in 40 chance, but you'll have a lot more crates than chests, and you will only catch the one kind of crate according to whether you're in Hardmode.
- Sell off the extras of all the other cool equipment you got in the process.