A 1 lamp series ballast can be hard to find, but it can be replaced with a 2 lamp series ballast. Here is how a 2 lamp ballast is wired to a 1 lamp fixture.
The ballast blue wires connect to one end of the lamp, the red wires connect to the other side. The common yellow wires are capped with small wire connectors.
Replace 1 Lamp Series Ballast with 2 Lamp Series