Board Thread:Wikia Discussion/@comment-27.76.82.26-20180211080037/@comment-28781449-20180211085406

grind shipgirls which have it as stock equipment.

Chitose/Chiyoda are common and give 2 of these by lvl50, it's the most optimal choice.

Another option is Shoukaku/Zuikaku (1 at kai) if you just happen to drop one while doing someting