The cuneiform sign za is a common use sign in the Amarna letters and the Epic of Gilgamesh.
In the 14th century BC Amarna letters, "za" is used in the spelling of the word for "mayor" (city, city-state administrator), Akkadian hazannu.
[1] For example, in EA 144 (from Zimreddi of Sidon), obverse, line 5: "man-hazzanu", Lú-Ha-za-nu.
Za has a fairly high usage in the vassal states sub-corpus of the Amarna letters.
For the Epic of Gilgamesh, the following usage is found in Tablets I-XII: ṣa-(79 times); za-(32); ZA-(15 times).