Do you not have the system CD/DVD's for your iMac? Did you check Apples website? Here is some older versions I found on Apples website.
QuickTime 6.3.1 for Mac
QuickTime 6.5.2 for Mac
QuickTime 6.5.2 Reinstaller for QuickTime 7.0.1
QuickTime 6.5.3 for Mac