This is the piece you're looking for, I think, for the outside corner. As far as where to buy it, I don't know exactly what you're looking for. Your best bet is to do an internet search.
http://www.tilestoreonline.com/Chair_Rail_Out_Corner_Montana_Gris_p/85-4l017-crc.htmI don't know if there is an end cap, I would probably make those by doing a return. Returns are made easily enough by cutting the last piece at a 45 degree angle flush with the corner, which will leave a wedged-shaped space which will be filled with a wedge of tile. Hope I explained that well enough, it's easier shown than explained.