Since the Simplex is taken mainly from Office antiphons, you can find many of the antiphons in the Liber Usualis, and thus also in the NOH that Jeff Ostrowski has helpfully put online. You would need to consult the index.

Office antiphons tend to fall into repetitive patterns, so even for antiphons in the Simplex not found in the L.U., chances are you can find something similar and adapt it.

The following discussion points to the existence of a Graduale Simplex accompaniment