I've used bulbtown.com. The bayonet base bulb that fits is called a Ba9S. The envelope, or bulb shape/diameter is G3-1/2. Interestingly, I could not find an 18v version, most are 12 or 14.5. There are a couple of 28v versions. The 4.7 watt version should be bright enough at 18v. The bulb is #356.
There is an 18v bulb but it is a G4-1/2 which is a little larger (1.06" vs 0.94"). I would be concerned about heat, working at max voltage, with a large bulb being too close to the lense cap.
These bulbs are less than $1.