Which version of the OS are you running ?
There is no download since the update to iChat 2.1 in 2004
Each version of iChat goes with a version of the OS (it also has minor updates that come with the OS updates)
Reinstalling iChat up to the Snow Leopard iChat 5 version is done with the Install disks
https://discussions.apple.com/docs/DOC-4861 (Reinstalling iChat 3 Through 5 in Users Tips tab above)
As the Install disk may have an earlier version of the OS on it (say for instance you are on Leopard and have updated to OS X 10.5.8 and the Install Disk has version OS X 10.5) you will need to run the COMBO version of the Update to the OS to Update iChat before using it.
With iChat 6 you have to use the Recovery process to download the OS again. However this will automatically be the latest version.
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.3)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously