I know where to find 5-digit ZIP code boundaries. Do 9-digit ZIP code boundaries exist? If so, where might I find them?
It's important to understand that ZIP+4 codes and even 5-digit ZIP codes do not represent geographic regions. ZIP codes represent groups of addresses or delivery routes. The +4 codes might narrow it down to a block, a building or single organization.
ZIP+4 data is available directly from the USPS (including a file that relates it TIGER) or from third parties who have taken the USPS data and repackaged presumably with value added. The USPS data does not include geometries. It will require processing on your part.
You're unlikely to find a free, up-to-date source of ZIP+4 data.
You can look up the specific 9-digit zip code on the usps site: https://tools.usps.com/zip-code-lookup.htm?byaddress
ZIP+4s are not polygons like a ZIP Code is. They are line segments. Any mapping of a ZIP+4 is based on the centroid of the line segment.
This will get you down to the street.