The Roadster was last available 8/15-8/24, selling out 8/25. There are 10 cars that have been gone at least as long - 8 of the cars that were skipped after the 1.20 update dropped, the Escudo, which was skipped when its accelerated turn in the rotation was due, and the Sauber, which sold out the same day as the Roadster. The Sauber and Roadster were set to be next in line when the skipped cars started appearing again.
In fact, two cars that sold out before the Roadster did in its first of two appearances have yet to come back - the F40 and the XJR-9.