Destroy pirate lord corvettes through the solo assassination missions. You get at least one high grade material per explosion, and frequently more.
A special POI type (rare one) with one of the cargo racks that do produce PI on a quite regular basis will be available for exactly one week on planet Karas 2. Even more - in Odyssey the restricted area of this site is somehow borked, so you don't even need to relog each 20 seconds. Probably not a good way to get them, but still works
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