There are two main options but both require that you have the Install Disk (iChat 3 through iChat 5).
OS X 10 .5.8 also uses Time Machine so that may be an option if you are making Backups.
If you used Spotlight (Spyglass) and it could not find iChat then there is a chance it is not there.
It should be in Applications (It always pays to look Manually just in case someone has accidentally changed the App name)
As you seem to think you have misplaced your Install Disk do you make Time Machine Backups ?
If so, go to the Applications Folder.
Use the Time Machine icon in the DOCK.
The Galaxy background should appear with the Applications window getting slightly smaller.
Use the Arrows on the right to go back until you can see iChat in the window.
Click on it to highlight
Click Restore (it will look like it is being "dragged" to the Front/Current Time window)
Failing that check what is in the Trash (double click it and it will open like any other folder)
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Lion 10.7.3)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
"Limit the Logs to the Bits above Binary Images." No, Seriously