You can create your own too, for instance in Audacity. If I remember correctly you can simply add some White Noise and then mess with that (original white noise is veeery noisy, perfect for "start" and "end" static, but no good as a background sound); maybe by lowering the dB levels, maybe doing a high/low pass filter etc.
I also have a stock sound I use, somewhere, I think I got it through a tutorial on how to create radio voices...if you don't find anything else, I can send you that (when I get home).
Wolfrug out.