Occasionally from a boss fight they'll drop a random purple, though mostly purple-12s are a totally randomized reward from beating some of the queue missions or a round of kerrat. Its not like other MMOs where you can farm Torvalds The Mindslayer and hope after enough runs he'll drop the Hellreign Phaser Array. You just run missions and hope the RNG is kind to you. That's why Exchange prices on genuinely good non-bound gear are so silly; they truly are random drops. If you're looking for particular gear, you'll get it faster if you either farm EC and just buy it, or farm dil and hit the rep/fleet stuff. The odds of actually getting that Mk12 [crtd]x3 beam array you want from a mission (let alone 6 of them), well you're better off buying powerball tickets.