Study music theory. Seriously. Great source of inspiration. I don’t know where you’re at, so here are a few recommendations:
-All of Schoenberg (harmony, fundamentals of musical composition, structural functions of Harmony, preliminary Counterpoint), most of which can be found legally and free online. I’m currently working through these. It is a project that will take one year at least.
-Persichetti 20th century Harmony
-Samuel Adler Study of Orchestration (Rimsky Korsakov’s book on this topic is legally free available, but does not provide the recorded examples in one convenient collection) .
-read classical music (Imslp) since the music of western culture is greatly influenced by its harmony and structure
And here is how I draw inspiration from those: each new harmonic, structural or textural tool learned demands practice. So set yourself a problem like using passing dissonance in deceptive cadence, and work out a solution: write the harmony, invent the structure and motivic material and you’re almost done with a new piece.
It could also help listening to other pieces if you need an idea what genre to take on next.
Many of my items originated from exercises (especially the more recent) portfolio