Major chemical houses do not produce Ferric Oxalate for the use of
photographers - we don't use anything like enough. I have no idea who
they do produce it for. Impurities in the produced (no chemicals are ever
absolutely pure) which may be irrelevant for the intended purpose can -
and in this case usually do - make it useless for us.
You are better off either making it yourself, or if you don't want to do
this, buying from the specialist suppliers. As you say you are not a
chemist I recommend buying.