This Wiki articles claims of a fatwa by Ibn Taymiyyah on obligation of Jihad against Druze, Shi'ites and other claimed heretical sects. What is the source of the fatwa? Can anyone quote the fatwa or provide a link to it?
Yes. At least he did for Druze and Alawites.
In the book Majmoo al-Fatawa (Compilation of Fatwa), which is a collection of fatwas issued by Ibn Taimiyah published posthumously, volume 35 page 161 we have this passage (translated by myself with my additions in brackets).
The late sheikh was asked:
Of the Druze and Nusayris (Alawites): what is their standing?
He answered: These Druze and Nusayris are unbelievers by the Ijmaa' of all Muslims, eating their slaughtered meat is not halal, neither is marrying their women; they are not even to pay Jizya; for they are apostates of Islam, neither are they Muslims, or Jews, or Christians, they do not believe that the five prayers are wajib, nor that fasting Ramadan is wajib, nor that Hajj is wajib, neither do they make haram what Allah's messenger has made haram such as (consuming) carrion and alcohol and others. Even if they say the two shahadas along with these beliefs they are unbelievers by the Ijmaa' of all Muslims.
Ibn Taiymiyah then goes on to describe the beliefs of Druze and Nusayris (another name for Alawites) and then goes on to say:
Commenting on ousting groups of the Druze:
That they are unbelievers is something that all Muslims agree upon; in fact anyone who doubts that they are unbelievers is an unbeliever like them; they are not in the status of people of the scripture nor the idolaters; for they are stray unbelievers and their food is not halal, their women are to be enslaved, and their fortunes are to be taken away. They are apostate heretics and their repentance is not accepted; they are to be killed wherever they are found; cursed as they are described; and they are not to be employed in guarding or keeping doors or keeping peace. And their scholars and saints must be killed so that they do not lead other astray; it is haram to sleep with then in their homes; and their companionship; and walking with them; and walking in their funerals if you knew about their deaths. And it is haram for the rulers of Muslims to do away with the punishments that Allah has decreed upon them. It is only Allah's help that is sought and on Him do we depend.
After some search I myself found some related fatawas, but yet apparently it does not seem to prescribe the Warfare kind of Jihad:
Shaykh al-Islam Ibn Taymiyyah said:
“Refuting Ahl al-Bidah (i.e. the Shias and the like) is a Jihad, to the extent that Yahya ibn Yahya said: ‘Defense of the Sunnah is more excellent than Jihad in the path of Allah.’”
Shaykh al-Islam stated in another book:
When some people asked Ahmad bin Hanbal that they felt uneasy about (criticizing people) by saying that such and such is this, and such and such is that, he replied: ‘If I were to remain silent, how would the ignorant ones know the authentic from the inauthentic?’
Similarly, the innovators who introduce heretical writings which oppose the Quran and the Sunnah, and those who innovate in matters of worship, then explaining their true condition and warning the Ummah against them is an obligation by the unanimous agreement of the Muslim Scholars.
In fact, when Imam Ahmad bin Hanbal was asked whether a person who fasted, prayed and secluded himself in the mosque for worship was dearer to him than a person who spoke out against Ahl al-Bidah (i.e. the Shias and the like), he replied: “When he fasts and prays and secludes himself, then he does so for the benefit of his own self. However, when he speaks out against the innovators, he does so for the benefit of the Muslims in general, and this is more virtuous.”
So it is clear that opposing the innovators is of general benefit to the Muslims and is considered one of the types of Jihad in the path of Allah. Since purifying the Religion of Allah, and its Manhaj (methodology), its Shari’ah, and defending it from their attacks and that of their enemies is a collective obligation–a fact which is agreed upon by the scholars. For if Allah did not raise up some people to repel the harms (caused by) others, then the Religion would become corrupted. Indeed, this type of corruption is even greater than the corruption resulting from the disbelievers conquering the Muslims. This is because when the disbelievers conquer the Muslims, they do not corrupt their hearts nor their Religion, except after some time. Whereas the innovators corrupt the hearts from the very outset.
(Majmoo al-Fatawa, 28/231-232)
The short answer is yes, and you can read the original text in Arabic. I copied the translated text (The English version).
Shaykh Al-Islam Ibn Taymiyyah (may Allah be merciful to him) said to refute the suspicion of Druze:
All Muslims are in agreement that those people are Kafirs and whoever has doubt concerning this fact is also a Kafir like them. They are not like the people of the Book nor polytheists, but they are straying Kafirs whose food is not allowed to be eaten and their women may be taken in captivity. Their money should be taken because they are apostates and their repentance may not be accepted but they should be killed wherever they are and be cursed as they were described. It is not permissible to appoint them as guards on the gates. Moreover, their scholars or so called reformers should be killed so as not to mislead others. It is prohibited to sleep with them in their houses, accompany them, walk with them or escort their dead people. It is prohibited for the Muslim ruler to ignore establishing the penalty prescribed by Allah to these people.