Eren Kruger mentioned to Grisha that the only propaganda-free theory on how Ymir Fritz gained the power of the Titans in the first place was that that she came across "The Source of all Organic Matter" by accident and the rest is history. This hasn't been brought back up again in the main series so far, so all of this is pure speculation given the few pieces of information that are out there.
There have been several ideas floating around that Titans are in some way plant-based. For starters, they are active during the day when the sun is out, and become inactive at night with larger Titans taking longer to shut down compared to the smaller ones. Another thing to take into account is that at several points during the anime, there are many shots focusing on flowers and other plant life. One could easily interpret those for basic scenery presentation, showing how the world is beautiful despite the constant violence, but I think they're cleverly disguised foreshadowing for the revelations regarding the "source."
Now with that being said, here's my guess on where this "source" could be located: It's buried deep within Paradis Island itself. We know that Paradis Island is a highly stratigic location for Marley due to it's bountiful supply of natural resources, and I think this "source" is the reason behind that. I don't think it was by chance that Karl Fritz chose that island to build the three Walls when he exiled himself. Despite all of his actions, he seemed to have some degree of admiration for his ancestor Ymir Fritz. After all, he named the Walls after Ymir's three daughters. It wouldn't surprise me if he also decided to exile himself to the exact place where the Titan nightmare began nearly 2000 years ago when Ymir came across the "source."
The topic regarding the "source" is bound to be brought back up again in the main series. Who knows, it might just blow our minds when we finally get to see it and what else it can do. It might even be a key factor to the "Invisible Paths" connecting all Subject of Ymir through space and time. What are your thoughts about it?