I always go to youtube for backing tracks. Whenever I search by genres, like, metal backing tracks, rock backing tracks etc, some interesting results show up ( i find the jazz ones the hardest). Also try the piano backing tracks. For me a piano and an electric guitar is a match made in heaven. If you become comfortable with the common time signatures, go for the odd ones like 7/4, 3/4 or my fav one 5/4.