If you have more than one Mac, and one of them is stuck at a certain version of iPhoto because it's no longer supported by Apple, you may have to install an older version on a newer version of OSX. OSX Lion, for instance, is restricted to version 9.4.3, so if you have another device with a newer version of OSX, you may also want to install 9.4.3 on that device, even though the App Store wants to install a newer version of iPhoto.
If you have a Time Machine backup with iPhoto 9.4.3, you could restore it from there. If not, you need the original software CD you used to install iPhoto on OSX Lion. If you do not have that either, there are other ways to get it, but this would be out of scope for this article. Reading DOC-4381 will set you in the right direction, however.
If you have the iLife '11 DVD, that's even better.
Also, note that this only works for Mountain Lion and Mavericks. Yosemite requires iPhoto 9.6. See the support matrix for more information.
If you come across version 9.4.7, do not install it!
This version only works on OSX Mavericks, but does not tell you during installation.