This small screw may be quite hard to find, you could try asking TheBookYard in Liverpool UK or Mac recycling. The 'standoff part that the screw fits into is actually fixed to the logic board so I doubt that will be available anywhere.
I don't think there is any real problem without the screw, the fan 'Might' rattle or vibrate a little at times but other than that it shouldn't be a problem to use the unit. My neighbor has a 2013 15" and wanted me to replace his speakers, when I opened it up there was only 1 screw holding one of the fans and if I remember right just 3 screws holding the logic board inside! It was still working OK!
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