A quick search for "Private Internet Access download" might yield results from software aggregation sites (like Softpedia, CNET, or FileHippo). While some of these are legitimate, downloading security software from third-party mirrors is generally bad practice. You cannot verify if the file has been repackaged with adware or trojans.
Before you can secure other downloads, you need to download the PIA client. Here is the safest way to do so: