There are many methods to finding Ancestral items; as typical for Diablo, they can drop from most activities or monsters in the game. However, I was pretty stuck on Torment I at first because I couldn't find an Ancestral weapon. I was trying Helltides with the Accursed Ritual boss, Blood Maiden, thinking that would be a good method, but they wouldn't drop. Barely any items would drop that are Ancestral.
However, there is a surefire way to get one: Infernal Hordes are a guaranteed Ancestral item drop in Diablo IV. This activity is fantastic anyway, as you can still get materials or gold if you desire with remaining Aether, and you will get tons of XP, which will benefit your Paragon Board progress early on. Just make sure to farm up some Glyph XP from Pits earlier to have them active for easier demon slaying.
How to get Ancestral Items in Diablo IV