It's almost a 1 in 5 chance per crate, so you should get one fairly quickly. You're looking for the Lavaproof Fishing Hook, an accessory that lets you fish in lava with any rod/bait. Use the Hellbait to fish for Obsidian/Hellstone crates.

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. Especially get Crate potions, brew your own if you have to.

Max out your fishing power in usual fashion (equipment, potions, moon, weather, time, luck).

You also need a Shadow Key to open the Obsidian Lockboxes.

Make a lava lake at your base, or any other place thats safer than fishing in the Caverns, much less Underworld.

(If you got the Hotline Fishing 'Hook' from the Angler, you can skip this, but that's a Hardmode award.) If you don't have a Golden Fishing Net, craft a Bug Net with hellstone to make a Lavaproof Bug Net. The flip side, however, is that you can grind them with lava fishing, which is involved but ultimately straightforward: 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. In 1.3, they were a rare drop from common imps.

Terraria lava charm