Roccandil: A few options:

- I often found one of Tainted Brain Matter, Blood of Ch'thon, or Ancient Heart in Treasure Troves (which require dynamite to blow up). Troves seem to spawn regularly in certain areas, though I haven't made a map of that. I think the Grim Dawn wiki may list areas with treasure troves, though; so you might find that info easily.
- Seems like higher-level Aetherial bosses tend to drop Tainted Brain Matter more often than other bosses (just as higher-level Ch'thonic bosses drop blood more often).
- The Cursed Smith in the Necropolis Gates area can convert those three rare crafting ingredients into each other (though I don't remember whether the blood or the heart switched to brains), so if you have an excess of one kind you can switch between them for just a cost in iron bits.
- As a last resort (which probably doesn't apply to you yet), if you dismantle Legendary gear, you seem to be guaranteed to get one of those three rare ingredients. I would only do this with duplicate gear with lesser stats you don't want to trade.

Navagon: You need tainted brain matter for various creations and side quests (and many government jobs too), but I haven't found a damn single bit of it since killing the Warden for the first time. Even subsequent kills doesn't yield any more.

I'm now level 53 and this is getting old now. I really should have found some in the Steps of Torment or somewhere... anywhere by now. But no.

Any ideas?

Roccandil: - The Cursed Smith in the Necropolis Gates area can convert those three rare crafting ingredients into each other (though I don't remember whether the blood or the heart switched to brains), so if you have an excess of one kind you can switch between them for just a cost in iron bits.