When you create a project in iMovie (which it forces you to do before starting), it has a folder in the project folder where the assets are stored. So find your iMovie project folder. They may be keeeping them inside a bundle, so if you don't find it, right click on the things in your project folder and select "Show package contents." I haven't used iMovie in several versions, and Apple likes to change these things, so the details may be different...