There's a few good choices. Church chorale books are generally not too hard to get your hands on, and 4 part chorales make great sightreading exercises (especially the extra challenge when the hands are far apart). Most of my sightreading ability was gained through sightreading Bach, however. The Bach inventions are great because no matter how slowly you play them, they still sound good. Right now when I feel like doing some more challenging sightreading I try out a sinfonia or maybe even a fugue (though rather infrequently). Finally, don't worry about being able to sightread at speed. Your sightreading still improves if you're pausing frequently to determine notes, don't worry. The best is just to sightread as much as possible, without worrying about what in particular you're sightreading. One time I decided to sightread one of the harder beethoven sonatas randomly (I think the appasionata?), and it went terribly as you'd expect, but it was still a good workout for my sightreading muscles. Have fun!