The standalone isight is discontinued and you will not be able to buy a refurbished one from Apple.
You should be able to buy a used one from ebay though. There are a couple of other USB webcams here and there that are OSX compatible.
To be able to use other cameras with programs like ichat, the main issue is compatible quicktime integration and the fact that ichat only recognises firewire.
As well as the few specific Mac osx USB cameras being sold out there other than isight, you could also look into the possibility of running a program like Macam:
http://webcam-osx.sourceforge.net/
If you want to use ichat with other USB cameras, then ichatusbcam software does the job nicely for many:
http://www.ecamm.com/mac/ichatusbcam/