If you really want an iSight, try ebay or craigslist. Would probably cost you 150-200, so doesn't seem worth it to be for a mini. Check out logitech, there are probably some $30-60 ones which would be fine.
I have heard the Xbox Live Vision camera is actually a really good web cam. It supposedly works with macs, although Microsoft doesn't tell you that. You get a decent quality webcam for $40 (IIRC), not a bad deal