The mirrored copy of the item is a bit less valuable than the original item, since you cannot change the mirrored copy in any way. This means that you will need to decide on gem colors and all that stuff before making a copy.
How to get a Mirror of Kalandra from dropsLike all other currency item, the Mirror of Kalandra can drop from any source in the entire game. This means that every monster, every chest, and every destructible object has a chance to drop it. There are no special mobs that has a much higher chance to drop it, but it will of course drop more frequently from higher level monsters than low-level monsters, so stack that item quality and item quantity increase to get better chances.
The mirror is about 1,000 times more rare than an Exalted Orb, so most people will never ever see one drop.
Getting a Mirror of Kalandra to drop pretty much boils down to pure luck, but there are also a few other options on how to get it.
Getting a Mirror of Kalandra from Divination CardsThere are currently a few Divination Cards that can give you a Mirror of Kalandra, and these are;
1) House of Mirrors: Collect 9 House of Mirrors to get a Mirror of Kalandra. This Div card can only drop in The Alluring Abyss, a level 80 area that contains the boss Atziri, Queen of the Vaal. It's a pretty expensive area to enter, and the drop chance for the House of Mirrors is extremely low, so it will be a very long, tedious and expensive grind to get a Mirror from this card alone.
The Div card can also drop from other sources, like strongboxes that contain Div cards, or other Div cards that give you a random Div card.
2) Emperor's Luck: Collect 5 to get 5x a random currency. These cards are not that expensive, but keep in mind that they have a weighted drop just like all other drops. This means that you have a 1,000 times higher chance of getting an Exalted Orb than a Mirror. And even getting 5 x Exalted Orbs from this card is extremely rare!
This Divination card can drop in the following maps: Courtyard, Orchard, Terrace, The High Gardens, and The Imperial Gardens. It is a pretty common drop, and you won't have to run many maps before you have a full stack of 5 of these.
3) The Void: Trade for a random Divination Card reward. Like with the Emperor's Luck, the rewards are weighted so that you get most of the common card rewards, so it's extremely unlikely to get a Mirror of Kalandra from this - but it's possible if you are very lucky!
This card can drop from any mob in the game, and it does not have a certain drop located where you can farm it.
Getting a Mirror of Kalandra from Mirror ShardsMirror Shards are a currency item that can be used to make a Mirror of Kalandra if you find 20 x Mirror Shards. These shards are very valuable by themselves, and they can only drop from Harbingers, or from areas affected by the Harbinger modification, such as Harbinger modes in the Azurite Mine, or when encountering them on a Zana mission.
This makes them very difficult to farm, but a lot of people come accross this shard from just playing the game, so it's a nice option to buy them a few at a time, and gradually save up to buy a Mirror of Kalandra.