Magento is open source e-commerce platfrom and can be found anywhere on net, however I always recommend that installation should be downloaded from official page: http://www.magentocommerce.com/download
When you install Magento to test something it is very convenient to have preloaded categories, products, products images and pages. Just as Magento insatlaltion, ‘sample data’ that include data base and images can be downloaded from official site (be sure to read manaul how to install and use sample data package): http://www.magentocommerce.com/knowledge-base/entry/installing-sample-data-archive-for-magento-1610