I had this same question. I have find things on the web that are free but the main problem that I am finding is that I may find an icon of a phone but when I find an icon of a email that I like it is different then the phone. The app starts to look like it was pieced together instead of having a consistent look and feel.
I thought I would first finish my app and then decided what I needed to do to give it a consistent look. by hiring a designer to create my custom items I need.