BBC has released a sound library with over 16,000 samples that can now be downloaded. You have to make sure you are abiding by their terms and conditions, but diving into this treasure trove of samples could unlock something you didn’t know you needed before.
There are pages of military gun sounds from previous wars, people walking and all sorts of animal sounds, so there really is everything.
Make sure you pay attention to their terms and conditions before using the samples and have fun with it all. Even if you don’t use a sample, going through these may inspire you to create something on your own that is similar or find somewhere else that sounds fresher and more refined.