Daily Lisa'tasksWorld bossNarrative Collection
Blood gems are the rarest currency in the game. There is no way to farm them at a rate greater than one per day outside special events like the current narrative dungeon or World Bosses. It can take many months to get 240 blood gems for just one skin.So my advice is that if you buy a skin, make sure it's worth it. I wouldn't get 3 stars Mona, Lisa and Davi costumes as you will never use these childs once you are past the rookie stage.
You could pay for them in the store right now. they are $5 usd for 25 right now.