Had a similar problem when I was shopping for tyres. Most guys had stock of the sizes I wanted in run flat only. However, I was advised that the E39 ride would be terrible on run flats and this coupled with the silly prices for the rft's, I decided to shop around a bit longer an eventualy found non-rft's in the sizes I wanted. However, it was a bit of a struggle ....
BTW, mine was 17s not 18s.
Have you considered other brands?