This is a complicated question. I have revised my answer a lot.
Basically, the following points will allow you to make up your own mind:
1. In terms of the determination of the source of thought, what genuinely determines anything is a free choice. Otherwise, we are saying that a thought might as well be a purple cantaloupe.
2. Now, there is another sense of the origin of thoughts, which is: what is the origin of a thought's SIGNIFICANCE? Here we can imagine an infinite progression of earlier forms of a thing. But at what point do we decide that it has none of the properties? Should we decide that the earlier thing is 'more pure'? Or that the later thing is 'more evolved'? We don't necessarily know how history works, so it is a big puzzle.
3. At this point you could argue that the source of thoughts is psychological: they arise from the justification that something (some mental event) was significant enough to be a thought. However, it could be argued that this view is not objective enough. For example, how is the origin of an interpretation the origin of the thought itself? They might have different origins.
4. Another view is that thought is a kind of object like any other object, with a significance based on what it IS, or perhaps HOW IT FUNCTIONS, and other such theories. Some of these theories see meaning as ALIENABLE FROM THE OBJECT (or thought), whereas others see meaning as INALIENABLE.
5. Some of these theories, in their efforts to discover the INTERPRETATION, actually lose track of WHAT WAS ACTUALLY MEANT BY THE OBJECT'S SIGNIFICANCE. In other words, it is cheating to think that there was a source for a thought, if we do not know what a thought is.
6. At this point a philosopher would argue that the source of a thought must be the source of an ABSOLUTE THOUGHT. Some philosophers would decide this is too difficult a question, and decide to be relativists. Others might take a religious view, or decide to become coherentists or materialists.
7. So, in short, it is complicated. One view of thought is that it is contingent, and essentially interprets objects which are already present, or even possibly occurs 'ex nihilo' like a divine creation. In this sense, the origin of thought is the process of interpreting whatever objects --- or no objects at all --- which already exist.
8. There are also other interpretations which favor essence, possibly providing a more universal answer than interpretation, or other views of inherency---yet another word---which favors concepts of how thought is formulated to qualify as a thought in the first place. I would say the second of these views is somewhat more popular now, but there may be other theories as well.
9. For example, the objective-coherentist view might favor the idea that thoughts can fail to exist simply out of the system that is being used to judge whether the thought exists. Under this view, there may even be thoughts which are not thoughts---a rather useful tool, when you think about it.
10. But even this view may be open to interpretations such as essentialism or materialism.
11. Let me play "devil's advocate": If someone insists that free will is all that exists, one cannot deny that free will is the source of thought. So, in this way, there is no doubt that there are multiple logical perspectives, because there is more than one explanation for the universal. What if the universal is complicated? Then we're even further up the creek. Simply because we live in the age of science does not mean that we get a simple explanation on every subject! I find this to be a good principle (although I hope you have found some clarity by considering my writing).
NOTES:
----I agree with the answerer called Mr. Purple somewhat, that a thought is it's own source, but I also think it deserves some clarification. I also agree with the answerer called Simply Dreams in that the Cogito ("I think therefore I am") has some subjective properties, and I agree with some of the unstated gist of Nameless's statements, namely that there is a need for modernism.-----