Hmm.

Or get him from Lucky Stars Event where he usually placed as one of the redeem items. These past months, Lucky Stars Wheel is always with Fukurokumaru's Deal so it is much better to get him from there in my opinion. Getting Training Potions and New Refine Optional Packs while spinning the wheel and having a chance to win the jackpot ninja, as well as getting some frags of ninjas also, then making a progress for your Fukurokumaru's Deals milestone and use the stars to redeem his frags. Though, it will be much better if you will do it when you have at least 25k already so you can go full milestones in Fukurokumaru's Deals and get another ninja.