Unfortunately for Nextel 7520 (and 7510) users, we are still trapped in the world of MIDI ringtones. While there are some decent ones out there, here's a site that catalogs a lot of nice beats.
Free Ringtones UK; Midi catalog.
VodaBerry; also known as blackberrytunes.com (despite the great sounding midi files, they are still midi which can be DL'ed onto your BB. It won't sound the same as if you played it off the PC but it will work)
Note: Reqwireless browser does not support MIDI playback but NOL browser does (so many page views just to get to the meat!).
What's nice is that with OS4.0, there is a built in MIDI player which was not present in the 7510 (running pre-4.0 OS). The MIDI player has a nice interface which features a play button, a stop button and a menu. Yes, you can save the ringtones. I'm using 'PinkyandtheBrain'. sweet.