These only start dropping in Torment difficulty, so you can't find them earlier. They are quite more rare than they used to be. The easiest way for guaranteed ones is to go for Infernal Hordes, which you need a Compass to trigger.
Farm up the waves and open the grand equipment chest for 200 Aether and you should find at least 1 guaranteed Ancestral and some Uniques that only drop from this activity.
How to find Forgotten Souls in Diablo IV
At the moment, they are fairly difficult to come by; they drop from Tormented Chests in Helltides (3 drop from regular, and 9 from Mystery ones). So that might be one of the more reliable ways of getting them. You no longer get them from salvaging, even Ancestral Legendary items.
How to find Tributes and Bargains
You can get Tributes from other activities such as Whispering Caches or Nightmare Dungeons. The drop rate doesn't seem great, even from Caches; perhaps Nightmare Dungeons are better for this.
You don't have to use a Tribute for the new Kurast Undercity dungeon (basically, a time-attack dungeon), but you won't get a lot of loot that way. You should come in with a Tribute, which can customize what kind of rewards you'll get, so you can tailor this to your liking.
Once you have a Tribute, you can additionally Bargain to get even more rewards.
How do I level up Paragon Glyphs now?