Thank you all!

My conclusions so far are these:

Brown will work - some recommend using more, some recommend using less. Will try both. Would be nice if I can find out the stoichiometry - will have a look at it later.

Next time I'll buy the pale brown, which seems to be the low-iron green under a different description. But I have enough brown stuff to last me a lifetime or so - 500g was the smallest package of "technical" grade, at less than half the price of 10g "Ultrapure"...

Ferric oxalate is not listed in any of my supplier's lists - under any name I can think of. But mixing Fe(OH)3 with oxalic acid, with a little H2O2 to maintain the oxidation state, should give ferrioxalate in solution. It may need a little extra acidifier, but I doubt it.
I'll also try importing some from the UK - if customs will let me.