LMK PWEASE

Anywhere, it's a % of it dropping when a leg drop. The more leg drop rate the most chances to get it, so high GR, speed TX rifts, cow lvl & vault are pretty much where you got the best amount of leg drops.

Rifts are easier for me

ACT 2 Dahlgur oasis

I find some decent legendaries (six to eight a game) doing Torment X rift runs with just a little magic find (45 %).
Usually one or two ancient versions every other run. That's all I really do for my farming. Public rift runs is good.

Just play on high difficulty (t7-10) or grifts similar and upwards and you should get one.
Best way to get one is by playing with someone who doesn't need and can share when he/she finds.