To find an Alternate Extension in the Unity Connection database, simply navigate to the Unity Connection administrative GUI as normal.
Limit your search to a particular partition.
Check the radio button to Display Primary and Alternate Extensions in Selected Partition.
Search by the Alternate Extension you are looking for.
Below is a sample search for 2465 that returns a user with a primary extension of 2466.
This is the user returned when searching for 2465. Note, 2465 is the alternate extension, not the primary extension:
Updated 2/3/2014: This is the page that would be referenced when searching normally for 2466, the primary extension for the user:
Updated 2/3/2014: This is the alternate extension page that is normally not displayed via the default search, but which matches when using the particular partition and Display Primary and Alternate Extensions in Selected Partition :