You can sometimes find authorized resellers selling old, unopened CS5 Extended retail boxes (DVDs) on eBay or Amazon Marketplace. Prices range from $150 to $800 depending on scarcity. Alternatively, Adobe Support may provide a CS5 download link if you already have a valid, registered serial number from 2010.
To understand the appeal of the "full version" of CS5, one must understand the landscape of the software industry at the time. Photoshop CS5 belonged to the "Creative Suite" era, a time when users could purchase a box containing a CD (or DVD) and install the software with a serial number. Once bought, it was yours forever (or at least until your operating system refused to run it). Adobe Photoshop CS5 Extended Full version