Turns out, Hardin has been made aware of the issues with the filtration system, and despite having been brought replacements by his procurement specialists, he has placed them in his desk, away from prying eyes in his efforts to overthrow McNamara. 

The items necessary to repair the air filtration system are now present in Hardin's desk. Be careful, however, as while the Brotherhood on the whole has no knowledge of their existence, these items are considered the personal properly of Hardin himself. Be wary should you choose to liberate them from their newfound enclosure. 

Note: The originals are not removed or otherwise replaced, so you can still acquire them the normal way if you so choose. I mostly made this because doing the fetch quest gets really, really old after so many playthroughs.