Various sources. Almost all of the early stuff comes from https://dova-s.jp, but nowadays I use a wider variety.
If I were to list all of my sources of inspiration, I'd be writing a book, but some of the more prominent ones would be Doki Doki Literature Club, Life is Strange, various mythologies and the many hundreds of anime and manga I've watched/read. Of course there are plenty of other games/VNs/stories/... that have inspired me to various degrees over the years, this is just a few of them.