The Perandus Manor map is one of the most valuable Unique maps in Path of Exile. It's a great map to run since it will always spawn Cadiro Perandus, but it's also very difficult to obtain. There are basically two ways to get this map without just buying it from another player:
1) Buy it from Cadiro Perandus for 2,500 Perandus Coins. Cadiro will sometimes sell this map for 2,500 coins, but it's pretty rare. It is almost always worth it, since 2,500 coins tend to be worth a lot less than the map, so pick it up if you come across it. He will only sell this map if you encounter him in an area that is a map from Tier 5 or above.
2) Trade 5 x The Mayor Divination Cards for it. Five of these cards will give you a The Perandus Manor Chateau Map. The Div card itself drops from any mob in the map Ivory Temple, and it's kind of rare to get. However, it's perfectly possible to farm it with some determination. The Div cards can also be bought and sold, so this is a great option if you miss a few to complete your set of 5.