The WTO Agreement is often cited to 33 I.L.M. 1125 (1994). This version contains some editorial deletions.
The NAFTA text is frequently cited to 32 I.L.M. 289 (1993) because it is a Bluebook authorized source.
The Bluebook (20th ed. 2015) has rules and examples for citing a variety of WTO/GATT documents, including panel and Appellate Body reports, in Rule 21.11.