I'm not that familiar with the film (it's been a while) but I'd expect they were most likely produced by the filmmakers themselves.

There are other examples I can name. In The Owl and The Pussycat, starring George Segal and Barbara Streisand, there's a scene where Segal is walking through Times Square and discovers posters for a (fictional obviously) porno film called Cycle Sluts, that stars Streisand's character. They actually shot a bunch of weird bikes and black leather stills with Streisand to serve as the posters outside the theater. See here.