When I am meditating, most of the time the thoughts feel like arising from the heart center, and sometimes they feel like lingering in the head. Same with the 'feeling of self', most of the time it feels like 'I am' feeling is located somewhere around the heart. I need to know as I think being mindful in this region will help a lot to stop thoughts and to attain 'anatta.
This is like asking where the source of the wind lies. Winds don't have a 'source'; they are mere movements of the air that come from everywhere and nowhere. A thought is a movement of attention, an eddy in the flow of energy that we commonly think of as 'being alive'. When the energy moves on, the thought disappears like a breeze fading into stillness.
But by all means, focus attention wherever you perceive an arising. That is a good way to learn the essence of emptiness.
I'm not sure why, it seems to be localised in the head. I think that's because of proximity to sense-organs, i.e. no matter whether thoughts are like words or more like images, the ear and eyes (which are in or at the head) are where we're accustomed to those being.
See what SN 22.100 has to say about the five clinging-aggregates:
It's just as when a dog is tied by a leash to a post or stake
If much of rupa (form) is perceived by sense-organs in the head then perhaps that is "where" we assume "everything" (including for example, "thoughts") arises.
See also the twelve nidanas where feelings and so on arise from "contact" and "sense bases".
Incidentally part of a (non-Buddhist) martial arts training is to move that centre-of-awareness downward into the lower of the three dan tien -- from that I assume that that location isn't fixed, inherent, or single, but is perhaps more a matter of attention and of habit. Also as I was taught it those aren't exactly a "source", they're more like a node of a network on which there's circulation.
And when a nice cloth is seen it's inside of that, right? :-)
The source, cause, is craving, caused by not-knowing (things as they really 'are', arise, decay).
Now, where is it felt here now, or running in wheels among the aggregates?
And which fool told good householder to try to attain anatta?