I mean other than the futility of looking for The True Source of morality outside yourself

The argument that morality must have a Source is conditioned by the way most people encounter moral exhortation, thtough religion. The idea is that moral principles are compulsions or obligations, which means, in turn, that they are something like edicts or commandments. Whilst there are lots of humans who can offer opinions on what is compulsory, they are all subjective opinions: no-one can lay down an objective edict. However, the basic idea of a "source" merges two separate ideas: who tells you about some item of knowledge, and what makes it true. The mathematical truths relayed to you by your teachers are objectively true, not because teachers are Objective Sources, but because of the ways they are proved an justified — which is not by the teacher personally. The set of things tha are true just because someone says so is small. Eye witness testimony, personal reminiscences, reports of subjective states: not many things work on the Source system, and there is no reason to assume ethics does.