If I may ask, are you certain that "source of everything that exists" does not exist in Buddhism?
"exist' is a tricky term.
In the buddhist context, "exist" often refers to a reductionist point
to which something can be said to be self-arisen.
In other words,
something exists only if it does so unconditionally.
Thus, an atom doesn't "exist" because an atom is made up of even smaller components which only play together temporarily. Thus, an atom only "exists" or occurs conditionally.
Actually, it is more accurate to say that no thing exists which is an atom
which doesn't mean that there is no such things as atoms, rather that atoms are not self-existent. Atoms rely on component parts.
Thus, to say that there is a source of everything that exists, one would have to begin with the premise that things (everything?) has some inherent existence, and this is something that Budhism more or less rejects.
the appearance of everything having an inherent, or self-arising existence is said to have a source, and that source is the mind.