I wouldn't even bother. On pretty much every single iMac since the very beginning, replacing the PSU is not for the faint of heart.
On the G3's you'll have a real hard time finding one anywhere, and you're dealing with a CRT as well, so risk of death is not out of the question. The G4s require some means of supporting the neck and display so they don't snap off and cause more problems. The G5s are a real PITA to get out, requiring you to practically break the case or take a whole crapload of additional parts out. Finally, the Intel based models tend to require some special tools for removing the glass over the display (save the original x86 iMac which was basically the same design as the iSight models and required a special access card to release some internal hinges).
And with pretty much all the Intel based models, the PSU is bare, so there's a serious shock hazard to contend with.
So even if you find the part, getting the old one out and the new one in will likely present a significant technical challenge. Anyone who knew how to handle this kind of repair would know where to get parts, how to describe the system, etc. So that's an easy way for me to tell you probably don't want to attempt this.