One source not mentioned is the extraction from barkeeper's friend and similar cleaners. Not exactly a chemistry reaction but a good reasonably safe
extraction (don't ignite the acetone).
The yields must be terrible though. From memory, isn't that stuff 5-10% oxalic acid?
The article I posted shows it is possible.
The actual amount in a 12 oz can is something more than 16.8g.
This is an expensive way to produce oxalic acid but it is interesting.
Keep in mind that rhubarb only contains about .5% so this is 10 times
as concentrated.