Surprisingly, there is no free reliable source for ZIP code data -- incredibly, the U.S. Postal Service does not make it available. They do offer PDF maps for a fee, but no GIS data. Third-party companies do make GIS data available, but they tend to be expensive.

Because we're working on a very limited budget, we use the Census Bureau's ZIP Code Tabulation Areas (ZCTA's), which are a close approximation, but unfortunately date back to 2000. This means that this data should not be considered too reliable, especially for newer areas.

Zip codes of Utah	Zip codes in the Salt Lake City area

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Origin

This data is available from the Census Bureau.

Abstract
A ZIP Code tabulation area (ZCTA) is a statistical geographic entity that approximates the delivery area for a U.S. Postal Service five-digit or three-digit ZIP Code. ZCTAs are aggregations of census blocks that have the same predominant ZIP Code associated with the addresses in the U.S. Census Bureau's Master Address File (MAF). Three-digit ZCTA codes are applied to large contiguous areas for which the U.S. Census Bureau does not have five-digit ZIP Code information in its MAF. ZCTAs do not precisely depict ZIP Code delivery areas, and do not include all ZIP Codes used for mail delivery.