NAFTA created the "North American Free-Trade Zone," comprised of Canada, the United States and Mexico. The treaty is codified in U.S. law at 19 USC §3301 et. seq.
The original NAFTA treaty is posted with the NAFTA Secretariat (choose "English" and then "Legal Texts") and at www.solon.org/Treaties/NAFTA/index.html. Another version is on Lexis (INTLAW;NAFTA).
The best single source for NAFTA materials is probably the multi-volume North American Free Trade Agreements by James R. Holbein and Donald J. Musch. In addition to the treaty itself, this set includes just about all the related treaties plus a lot of other related materials.
The NAFTA Secretariat posts the NAFTA treaty, Rules of Procedure and Decisions. More primary materials are available free from WorldTradeLaw.net
Lexis has lots of NAFTA materials, including NAFTA Panel Review Decisions (INTLAW;NAFDEC).
For more information and resources, see Basic Info and Online Sources for NAFTA and CAFTA Research by Francisco Avalos and Maureen Garmon. A bibliography of NAFTA-related materials was published in a 1993 issue of The Record (v.48 p.657). The Harvard law library posts a NAFTA Research Guide.