If you have a PowerPC iMac, purchase a new, secondhand, or refurbished Intel one.
If you have an Intel iMac, phone the online Apple Store and order a Mac OS X 10.6 DVD.
If you're not sure whether your iMac is PowerPC or Intel, choose About this Mac from the Apple menu. Note that all new Mac models released in the last six years are Intel.
If you don't currently have iTunes version 10.6.3.25 installed, try updating to that rather than 10.7.0.21.
A .dmg installer for iTunes version 10.6.3.25 for Mac OS can be found at the following download page on the Apple website: