Amazon.com has quite a few 3rd party sellers with used iSights - but prices are often 2x what they originally cost before the iSight was discontinued.
One possible alternative is a Unibrain Fire-i DV firewire cam.
USB web cams are such a ****-shoot on the Mac - so few models 'Just Work' so I set up a website
www.mac-compatible-web-cam.com just to address this issue and help people find known Apple Macintosh OSX USB webcams that do. Webcams that are 'UVC-Compliant' are just plug and play - no drivers needed - on OSX 10.4.9 and above.