This is an OEM replacement part,it is sold individually.
The purpose of the Air Purge Bulb is to purge all of the air from the carburetor.
It then brings fuel up through the fuel line in the tank into the carburetor.
Once the carburetor is full of fuel the Bulb will force the excess fuel back into the tank through the fuel return line.
Over time the bulb will loose it elasticity or begin to crack, it will need to be replaced at this time.