Sorry, could not find the gasket by itself online, however, here is a link to a complete assembly:
https://www.cjponyparts.com/ford-doo...4-2004/p/LDC7/
You may try a salvage yard for a cheaper alternative. I believe that this was sold as a package by Ford along with the lock and was part#XR3Z-6343262-BA.
Hope you can either find what you need or make it from a gasket.