I have noticed that on both Paizo's PRD and the d20pfsrd.com site that the source for Alternate Racial Traits is not always listed. For everything else like Feats, Spells, Archetypes, etc. this information is all listed.
The problem is that my son and I are considering joining PFS which requires that you own the source that your character's material comes from. I am likely going to be needing to make several purchases (mostly PDFs hopefully) but I need to know what source material each resource comes from. Is there some good method of finding this information for traits that I have not yet found?
Nevermind. I was directed to the Archives of Nethys which seems to have exactly what I was looking for.
Glad you found Archives of Nethys, it's a great resource for figuring out options. Most of the Alternate Racial Traits are from the Advanced Race Guide. Keep in mind there is a difference between Racial Traits (variant racial features) and Race Traits (a subtype of Traits, alongside religion traits, combat traits, etc.). The two are not interchangeable.
Some of them I was looking at were also in the Bastards of Golarion book. There are several other supplements like this for other races. But yeah, APG and ARG mostly. I already own APG. Going to need to purchase ARG.