Rare Engineering components are provided as mission rewards and in High Grade Emission USS's, and its apparent that these are more frequently found prior to unlocking the relevant Engineer, so include the grind of low paying missions for Exquisite Focus Crystals, Modified Embedded Firmware, Pharmaceutical Isolaters, and Imperial Shielding asap as the game's RNG is programmed to provide less of these items when you're actively looking for these items.
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