Customsforge is a website that you can search to find Rocksmith files. Those files are either p.psarc for PC files or m.psarc for Mac files. So if you have a Mac running the Mac version of Rocksmith you can easily search Customsforge to find songs - and it even has filter features so you can find only m.psarc files if you like.
One note: If you want to play rocksmith files/songs, but don’t have Rocksmith or want to get Rocksmith, you can use the program Tonelib Jam. It’s actually more expensive that Rocksmith and doesn’t have any game features - but is an excellent practice tool with all kinds of things that Rocksmith can’t do like display tab or regular music notation, record yourself, etc. One of Tonelib Jam’s killer features is that it can natively read Rocksmith files for Mac and PC, GuitarPro, and many other file formats. More info on this thread.
Full instructions for how to use custom songs (CDLC) are here: