This site has a lot of audio books http://www.gutenberg.org/ I had to use it when I was in school. Also I think its better to first be able to understand people speaking fast than it is to understand complex sentences. Another good way to practice is to listen to short stories and watch short news videos.

Try to find somenthing that is a little bit above your current level. You're right that you won't progress as fast if you stick to material you can easily understand, and that you'll also have a difficult time if you choose something too difficult. Ideally, you find the sweet spot in the middle, where you don't understand everything, you're slowly figuring out the meaning of new words and expressions, but you understand enough to follow along. It can be hard to find just the right level of material (that is also interesting to you!). You're the best judge of your current level, so you may have to just jump into interesting-looking books at random and see how well you do. If a book is too hard, save it for when you are more advanced.

You could consider slowing down the audio, the playback setting often offer this option. When I did it years ago, I opted for light entertainment radio programs, where it wasn't a big deal if I missed something, compared to the long term exposure to it.