Best place to farm Heart of foreign god? ):

Tomorrow's hunting quest has the best available drop rate.

Event wise tomorrow hunting event. Normally you just hope Demon appears during 30 AP and 40 AP caster training ground. For free quest they will only drop after Shinjuku EoR update.

For future event: -
Christmas 2018 - 10 in shop
Moon Goddess Rerun - 10
Guda Guda Honnoji - 10
Guda Guda Meiji - 10 as Sengoku point rewards
Seraph - 5 as mission rewards.

yeah, farm tomorrow or the events, dont do like me, on friday caster quest.... weasting 20 apple for 4 hearts ....

Caster training ground is the only place to farm them outside of events. They don't appear on any free quests presently.