There are a lot of free ones available over at IconShock
that you can use in your projects. They give you a lot of sizes and formats in each set. There is even vector sources available for most of the free ones. (those are free too)
I am not too sure what you would do with most of them, but some of them are quite usable. There are many that could be used for a media player application.
The way to get all of them is to swap them with others on their forum, so that way you get everything. This can take a bit of time. Downloading takes even longer since they will only allow you to download one set at a time and they don't support download managers.
If you want to see some of what they have without signing up, take a look at the right side
of the page.
There is currently a total of 127 different sets. I have managed to download 92 of them over the last year (getting the rest today), and they are taking up over 6 gigs on my HD.