I want the supplemental address fields to be searchable under at least "advanced search". Right now the text entered in "supplemental address" can never be used for searching and retrieving. how do i go about it?
It does sound like a reasonable feature request, and the good news is that it's already been implemented.
The pull request was merged in March, so any version of CiviCRM from 4.7.18 upwards should include it in the advanced search criteria.
I checked locally and here's a screenshot of the changed address fields in advanced search: