I do not believe anybody made one in 1/18 yet. The one you have seen must have been a conversion based on a Bburago. Bang did make a 348 Spider in the 1/43 scale which is a nice and relative cheap alternative. There are many people doing conversions on Bburago 1/18 cars but most of the time they limit it to another color or a race version. I believe someone in Germany mad the 456 Convertible, but I do not know who.
Niels
Niels