I'm looking for a word that means without a source but that is a little shorter and simple than sourceless. Coulnd't find any on Thesaurus.
here is a suggestion:
aseity - the quality or state of being self-derived or self-originated
or try groundless or just source-less if it gets complicated.
There are words like groundless and unfounded that basically mean that you don't have supporting facts but not exactly the same as sourceless.
In the absence of more context, I suggest "self-manifested".
I'm a bit prejudiced to the concept of fons et origo but a hapax legomenon is a term of which only one instance of use is recorded and anything without an origin is cryptogenic or a term that refers to something of obscure or unknown origin.