With a check box format (and 50 check boxes in one field is a lot of check boxes, other options might make more sense) the question arises as to what type of "find" you want to do:
You can find all records that have all of the selected states. If you select "California, Oregon and Nevada" you find records that have at least "California" AND "Oregon" AND "Nevada" selected.
You can find all records where at least one of the specified states is selected. Find each records where California OR Oregon OR Nevada is selected.
Both are possible, but the script used is different.
You can find examples of both scripts here: Scripted Find Examples