Forget the service station. They won't have the part, They'll won't want to mess with a piddly job and they are likely to overtorque it when they put it one.
The piece you are after is a called a petcock. Order one for your particular ped. You will need to disconnect the fuel line from it. (it could have a clamp on the fuel line or just pushed over barbs). The the petcock will most likely unscrew from the tank (counter clockwise but remember you are working upside down here.) There could be a filter or screen that comes out with it Replace it with good stuff.
Do this in a well ventilated area. Try not to get too much fuel on you and don't let anyone smoke Depending on the condition of your hose, you might want to replace it too.