I got mine through a crate, so I'm guessing the flag is randomly dropped.
(Update: other people reported different sources as well, so definitely random. Thanks to Sparechange49 here for the heads up!)
For those of you curious and interested in trivia, the flag is a solid yellow with a red S next to it. This also happens to be the Special Flag found in Namco's classic Rally Racing. So be proud you got a piece of history! ;p