Funny thing, this is my headcanon on where the SDM's youkai food comes from: Sakuya gets condemned criminals delivered from the village.
I think this is actually a reference to how in actual touhou canon the vampires such as the ones at the scarlet devil mansion are given humans from the outside world to feed on by other high ranking yokai due to the vampire uprising event and the negotiations that sprung in its aftermath. I believe this is done so that they dont attack the humans in the villagea in gensokyo itself, hense the humans they're given being from outside of the place through being spirited away. The picture itself is I think sorta comedically depicting a delivery to the scarlet devil mansion and sakuya picking them up for remilia and flandre. Luckily apparently those two are so small that they almost never actually drink enough to kill someone if I remember right.