iPhoto is free and available for downloading on the Apple website, as of 2015. To download iPhoto, go to the Apple website, and click Support at the top of the page. Click Downloads & Updates in the Resources section at the bottom of the page.
On the Downloads page, click the Browse by Product button. When a search text box appears, enter "iPhoto" in the text box, and click Search. As of November 2015, the oldest version of iPhoto available on the Apple website is iPhoto 4.0.3, which is compatible with Mac OS X 10.2.6 or later. The latest version available is iPhoto 9.4.3, and it is compatible with Mac OS X 10.7.5 and 10.8.3.