A 1 lamp rapid start (series) ballast can be hard to find, but it can be replaced with a 2 lamp instant start (parallel) ballast. Here is how a 2 lamp ballast is wired to a 1 lamp fixture.
Ballast wire colors can vary depending on the type of ballast or country of origin. Below are the same diagrams with different wire colors. The ballast from this example is from a desk lamp.
After 1 Lamp Rapid Start Ballast is Replaced with a 2 Lamp Instant Start Ballast
2 Individual wires, 1 common wire
Lamp Series Ballast with 2 lamp Parallel