If you are talking about Porsche 'Dark Blue' which is a very dark, almost black blue then I think like the OP said that is going to be very difficult to get.
AFAIK that was only used on the early Carrera models.
I would talk to Tom at Werk924 in Germany. Not cheap but they are really good at finding correct interior fabrics. Especially for rare models. Don't be put off by the 924 focus on the site. Tom is a real enthusiast, he helped me find the original blue Berber tweed for my cars interior. The site is a little bit tough to find your way around at first, and Toms wife speaks decent English if you want to call.