Where can I find sources for the narration that if a person has not recognised the Imam of their time they die the death of Jahiliyya?
This Hadeeth has been narrated in many wordings but with the same meaning. This called Tawaatur Ma'nawi تواتر معنوي.
In Saheeh Muslim , V. 6, P. 22 Kitab al-Imaarah : Who dies without allegiance in his neck, will die a death of Jahiliyyah.
In Musnad Ahmed Ibn Hanbal , V. 4, P. 96: Who dies with out Imam, will die a death of Jahiliyyah.
In Kitab al-Sunnah by Ibn Abi 'Aassim , P. 489: Who dies with out Imam on him will die a death of Jahiliyyah.
The Hadeeth is also narrated in the most authentic Shia books of Hadeeth like al-Kaafi, V.1, P.276 and in many other books. It means that the meaning of this Hadeeth is Ijmaa' (unanimous) between all Muslims.
Wassalam.