As I understand it, Ferric Oxalate is not a straightforward thing to make. According to Mike Ware (who should know) it's an, "'ill-characterised' substance, that is to say - not easy to prepare as a solid, of dubious purity and a highly variable composition depending on its method of preparation." B&S know how to make this stuff, and how to make it consistantly and of sufficient quality for alt processes.
Those chemicals that I can source locally I buy locally. But for Ferric Oxalate (and the Pt/Pd salts) I go to B&S. I know I'm going to get good materials that way.