Did you mean "the source of indeterminism is non-physical"? I'm not quite sure I follow this part. Thought I did but smoked a cone and read it again, now I don't.
So indeterminism is when something behaves in a way that isn't caused by the past. So like whether a radioactive element has decayed or not, quantum mechanics predicts a probabilistic timeline for its decay but until we observe it somehow, it's said to be in a super state. When we observe it, it takes on either the decayed or non-decayed state. If we accept an indeterministic interpretation of quantum mechanics (not all are) then we're also committed to saying which state it takes on is not dependent on the past beyond some probability distribution. Like if we knew absolutely everything about the universe now, we still couldn't say for certain which state a decaying element will be the next time we observe it.
But when observation happens and a concrete state is realized is the only juncture where the future can be changed, because everything else is deterministic (this is actually a tautology, put differently "all things which are not indeterministic are deterministic"). So what is it that decides which state is realized? I don't know, I'm not a quantum physicist, maybe the question is malformed. But one thing we can say
isn't the answer with certainty (based on our assumption of indeterminism) is anything in the past (the physical world). If the state a decaying atom is in depends on something I did then the world is deterministic, if it doesn't then I can't have any effect on it. Thus indeterminism gives us no more of a possibility of free will than determinism does.
