Creating Visual Novels is awesome! For the artist however there is a lot of work. Before creating comics I use to make visual novels as a hobby using the renpy engine and then moved on to gamemaker studio 2.
Here are some sources you can find useful for making visual novels for a serious project:
My commission page
https://www.yonseo.com/commission/
Sketchfab
Check the store tab and click on architecture, you can find everything from realistic to cartoony. Very useful for visual novel backgrounds.
Music:
soundcloud.com/artificial-music
Public Domain music.
Visual Novel Sprites:
When you commission you definitely have to take into consideration about using the work commercially. When you commission art you do not own the rights to the work. Some artists have contracts however I find that selling a separate license is much easier than contracts. There's no back and forth on what you can do with the work, contracts or royalties, etc. You would only need to purchase a license when your project is ready for commercial use.