You can use ITA to get the rules of the fare, if it exists.

a::ua /f bc=L
b::ua /f bc=L

Make sure that 'Check seat availability' is unchecked, and run the search. Pick your flights, and then on the fare summary page, there'll be a button for 'details'. Click that, and then scroll to the bottom where the fare breakdown is, and click on the fare code. That should bring up a page with the fare rules.

Also just because there is availability in a certain fare class, there may not be a published fare that books into that class, making it essentially unavailable.