Bob Norton sells many "fake discs", which are SGU files that correspond to the songs in many of the most popular fakebooks you can purchase.
Also, there are a lot of free SGU files available on the web. A Google search will help you find them; although, be aware many of the links are old and my not resolve properly.
Over the years, I've downloaded probably 50,000 or so SGU/MGU files (although many are duplicates). Even with the duplicates, I probably have found 20,000 or so unique files on the web.
Also, don't forget that you can load a MIDI file into BIAB and it will usually do a pretty good job of figuring out the chord progression of the song based on the content of the MIDI file.