I don't have a particular library that I can recommend, but on top of finding some good free stuff on Youtube you can always do the classic thing of breaking vegetables on mic and mixing those real quick to make some horrible horror SFX (in the good way, seriously). Celery in particular is a common one for emulating the sound of breaking bones.
Every time I join a jam, I always forget how long it takes to make assets, even if it's in 2D and setting them up. My unfamiliarity and stubborness with using Godot certainly doesn't help, but I'm getting there!
Awesome progress; keep it up!