I am not sure why folks are assuming you would need to go single speed. A broken shifter will not make you need to go single speed if you have front and rear derailleurs. A broken rear derailleur or derailleur hanger will but with a broken shifter you can still shift the other shifter. It isn't that bad to have one shifter if the other one is chocked into the middle gear either with a knotted cable, the limit screws, or even a improvised wedge.

With a broken derailleur or derailleur hanger a spare shifter does no good and I think the derailleur and hanger are way more likely to be a problem than a shifter.