I'm looking for any info on Witch Hunters that I can find on websites or older edition books. Where to look? Or you could also post some info that popped out of your head!

There are little bits and pieces scattered throught some of the past books but nothing that I can think of that really fits what your looking for.

Personally I'd recommend getting C.L. Werner's Matthias Thulmann: Witch Hunter book.

A witch hunter features as a villain in the Death on the Reik series. Most everything else, off the top of my head, is out of print: the Witch Hunter's Handbook, Condemned by Fire... that's really about it other than the odd appearance here and there. Also the old Warhammer Quest Witch Hunter character pack (but that's pretty divergent background; magical amulets and all).

edit: the Death on the Reik books look like they're OOP now too, though I'd imagine they could see an Omnibus at some point.

another edit: the first Warhammer Online book (Empire in Chaos) features one as a main character, though it's an alternate and future version of the setting from WFRP 3's standpoint.

Are Witch hunters' organisation independent or are they in sole service of the Emperor?

I enjoyed the Witch Hunter's Handbook, but I'm not sure if it's available anymore. Maybe you could buy it used from someone?

As already mentioned The Witch Hunter’s Handbook and the Mathias Thulmann omnibus are good sources of information on Witch Hunters.

Witch Hunters are also featured in several 2nd edition books. The career description can be found in the Core Rulebook as well as in Career Compendium (with a couple adventure seeds).

A few paragraphs on them can be found in Sigmar’s Heirs (p. 31).

Tome of Corruption Chapter IX: Defenders of the Empire (p.123) takes a bit more detailed look at Witch Hunters. It describes their history and tactics and gives a few ideas on adventures revolving around them. It also describes Renegade Witch Hunters, meaning Witch Hunters who are not Templars of Sigmar. A new career, Cloaked Brother, is also featured. These shady characters are a splinter group of the Templars of Sigmar who focus on gathering information and spying.

I believe Realms of Sorcery has more extensive information on the Witch Hunters as well.

The adventure Ashes of Middenheim features Ordo Fidelis, a secret society of Witch Hunters working within the cult of Sigmar.

The Thousand Thrones campaign has two Witch Hunter NPCs in very minor roles.

Also, I have featured Witch Hunters in two fan supplements I have written. You can find them here and here.

Thanks! Those were pretty usefull, particularly Tome of Corruption and Realms of Sorcery. I didn't found Witch Hunter's Handbook anywhere, though

Yeah, I'm not surprised. It's pretty expensive on Amazon's marketplace: http://www.amazon.com/gp/offer-listing/1844164071/ref=dp_olp_0?ie=UTF8&qid=1271479475&sr=8-1-spell&condition=all

For a good source on real life witch hunters/demon hunters, read the Malleus Malificarum (eng. The Hammer of Witches) - written in 1486 as a sort of handbook for how to identify, and deal with, witches.

Plunder it for Warhammer gold, if you are running a Witch Hunter campaign.

Another good source on real life witch hunts is The Spanish Inquisition by Joseph Perez.