Diablo 4 Baal, Infernal Hordes and Lair Bosses can all help you find the Charms your build needs
Do not salvage your badly rolled Unique Charms either, because there is a way to fix them.
If you are still missing the Unique Charm your build needs, Diablo 4 Season 15 has a few good places to farm them. You can get good results from Baal's Capstone Dungeon, and Infernal Hordes and Lair Bosses are worth adding to your farming rotation too.
The Best Places to Farm Unique Charms
Baal's Capstone Dungeon on Torment 10 or higher is currently one of the most recommended farming spots. Not only is this place really good for Splinters and Gems as well, but you can also get plenty of Unique Charms from repeated runs. Keep in mind that the T10 Capstones for Baal, Diablo, and Mephisto are equivalent to T12 difficulty, so you do want a comfortable build before you farm those.
If you prefer Infernal Hordes, it can be another good source of Charms, but only if your build can comfortably earn enough Aether to open the chests and get a lot from them. Lair Bosses are also worth farming, especially if you already have plenty of 
Lair Key to spend. You can target farm the Boss that drops the Unique you need as a Charm, and once you have enough, try to craft it through the Cube. You can turn a unique item into its Charm version if you have spare Unique charms that you do not need.
It is still random, so the best option is the one activity you can clear quickly and consistently. There is little point in struggling through Baal's higher-difficulty encounter if you can complete a ton of Horde runs in the same amount of time. None of the content really has a higher chance of dropping Unique Charms specifically, sadly. Hordes are usually longer to clear, Belial is the fastest boss kill, and you can pick the loot you want, and Baal is the fastest Capstone Dungeon.
Reroll Your Unique Charm's Affixes if Needed
A lot of people forget about this, but you can reroll the affixes on your set and Unique charms through the Horadric Cube. There is a recipe called "Horadrice Reroll" that does not affect the item's power or set bonus, and just rerolls the affixes on it. So if you finally found the Charm your build needs but its stats are terrible, you can keep it and just reroll for something better.
Each reroll requires 
Pure Primordial Dust 
Raw Primordial Dust and 
Infused Horadric Resin, so getting the perfect combination can become expensive. What usually misses is the Resin, so make sure to salvage unwanted Charms and Seals at the Occultist for more. Keep in mind that if your Charm has a Greater Affix on it, you will probably lose that when you reroll, so do not use the recipe on a Charm that has a GA you really want to keep.