Where do I farm a Banished Lords Talisman??

It's in the general unique pool now so any boss can drop it.
Lair boss, greater lair boss, infernal hordes+chest at end, astaroth, etc

your best bet is to cube an ancestral item with remove affix until its white and then cube it to unique until you get what you want

Is there a way to change the charm to an item?

i dont think you can reverse the charm to an item

Okay just wondering

if you run something like hoards or greater lairs you will see a lot of the random pool uniques