With internet access you should be able to download lots of songs to listen too?
How about starting with a list of the top 100 songs... there are lots of great riffs in these songs from Layla to Sweet Child O' Mine... how about Black Dog?!?!?
http://www.rocknworld.com/specials/top100/songs/
100 songs not enough?!? How about the top 500 songs?!?
http://classicrock.about.com/library/misc/bltop500a.htm?PM=ss13_classicrock
Good luck with your search for riffs!