The fastest way to find the song you're looking for is to google it using the keywords "SONG TITLE" Guitar Pro Tab.
Below you'll find a short selection of sites that offer free tablatures from a great number of artists. Make sure that you select Guitar Pro or Power Tab format tabs for download.
If your browser asks you whether to save or open the file, choose save. Otherwise the tab is saved into a temporary folder that might be automatically deleted.
Once you have saved a tab on your hard drive, you can add it to your Go PlayAlong library.