Best place to farm low req Oni Blades?

You have 2 options. Jaya Bluffs and Haiju Lagoon. Either map have just a few oni spawns spread across the area.
Now here's the thing, the gold droprate on starter islands in general is low but even more so in normal mode. And when you narrow it down to one specific skin drop from one specific foe, your chances of finding what you're looking for is just incredibly slim.

You can try but to be honest these are'nt really farmable for legitimate players unfortunately. Best of luck if you're gonna attempt it though, you will need it.