There is no mention of an official Superman emblem font, although some fans have created their own, including the Kryptonian and Smallville "fonts" available from Superman Homepage. In any case, the emblem has changed significantly through the years since its conception in 1938.
For example, the 2013 Superman emblem for the movie, "Man of Steel," features a highly stylized red "S" with a large and sharp upper serif, and an almost non-existent lower one. This figure is surmounted by a shield-shape border. By comparison, the first Superman emblem consisted of a basic sans-serif red "S" inside a yellow police badge shape.
The emblem in the comics became largely standardized between 1944 and 1996, with a straight upper serif and a rounded lower one, prior to Alex Ross's "Kingdom Come" story, which used a red on black and very heavily stylized zig-zag "S."