At present times you cannot get power / skill transfer cards other than the Limited items shop or limited offer packs. You have to decide carefully and intelligently how you use them cause once used, it's gone.
There is a chance that you can get skill transfer cards other than the shop back in 2017 or 2018 called the "DAILY SPIN" but Nanoo stopped it for some reason like other goodies like Daily gifts. What a shame... I miss those days