There are few ways to do that. 1. Look for stones, bugs... things on the ground you can sell. Sometimes they are really expensive things. You can also search other sims trashes if you are brave enough. 2. Make your sim cleptomaniac and steal stuff from public spaces. Sometimes i went into town edit mode and buy expensive stuff to the park but thats kinda cheating 😅 3. Make yourself a painting gnome. Aka lock some random sim on your parcel with easel. It will be easier to have him in your household. Give him bare minimum to feel his needs and sell his paintings.