Currently working on exotic sword. I've heard rocketyard is a good location, but haven't had much success.
Edit: Finished! I got bolt caster!
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Spin metal spawns in abundance. When I did mine I farmed the spawn location and the moth yards. They're close by so you don't have to run a marathon between each location. Usually by the time you've cleaned out one area resources spawn back in the previous. Rocket yard is great too since you can pop a 3oC against taken ultras while you grind.