Headhunter is probably the best belt for most setups in Path of Exile, but getting your hands on one of these belts are very difficult. The reason people want this belt is because of its unique effect: "When you Kill a Rare monster, you gain its mods for 20 seconds", which is very powerful!
The problem with Headhunter is that it can only drop in areas that are affected by the Nemesis Mod, and apart from Nemesis Leagues, the only way for this to happen is by having Zana at level 7, and spending 8 Chaos Orbs to activate the Nemesis Mod on a map. Once that is done, any mob in that area can drop the Headhunter belt, but it is extremely rare!
However, you can also use Orb of Chance to get a Headhunter, and most of the belts are actually "created" this way. You can use Orb of Chance on any Leather Belt in order to get Headhunter, but you must do this in an area with the Nemesis Mod. This basically means another 8 Chaos Orbs to Zana, and bring as many Orbs of Chance and Leather Belts as possible. Again, it is extremely rare, so be prepared to spend a few thousand Leather Belts and Orb of Chances.
A third way to get it, is by trading Divination Cards. You can trade 8 of The Doctor for the belt (can be farmed in Spider Forest Map), but these cards are also extremely rare and expensive. 11 of The Fiend will get you a corrupted Headhunter (can be farmed in Shrine map), but these cards are also very rare. Also other cards like The Valkyrie and Jack in the Box can give you Headhunter, but this is very rare.
As with any other items, you can buy Headhunter from other players. There are usually many for sale at Poe.trade, but they are very expensive!
During the Legacy League, you can chance and find Headhunter in any area that is affected by the Nemesis Leaguestone.