Good luck. I attempted to find a suitable camera last year for my grandkids. There are hundreds of model available using USB, unfortunately, most will give only marginally acceptable results when compared to Firewire. And any FireWire camera you find will be expensive, those hat have them consider them made of gold!
Just make sure it is compatible with a Mac and that usually means no 'drivers' are required.
That being said, if the mini has USB2, a suitable USB2 camera may work just fine (I had to find something that would work with FW400/USB1).
However, here are some links that might help:
<
LogiTech QuickCam© Vision Pro> Seems acceptable by some Mac users (including one with a mini. Around $130.
<
Eyeball Mic/Web Cam> I've never owned anything from this company, but from what I've read, I wish I'd heard about this camera last year. They make great products, especially from the audio side. Check all the specs/gallery/reviews/manual links.
Around $90.