In addition to the exhibition opening in June that I mentioned recently, there's another museum show currently up in London that features Church's landscape oil sketches. The show has about 30 small works, together with the large finished canvas Niagara from the American Side are part of Frederic Church and the Landscape Oil Sketch, at the National Gallery in London. The exhibit will continue through April 28 and then travel to Scotland.
Book: Frederic Church and the Landscape Oil Sketch
Wikipedia visual list of Church's works.