If you have the mix % then I would think pretty much any supplier would be able to mix it for you, just find your local supplier and talk to them - if you have a part that you can take in as well then that would be extra help (glove box door, ash tray). The trouble is that modern paint will mix up slightly differently even with the correct 'recipe', so they will probably need to tint slightly to get an exact match.
On another note.. 54 cabrio? lets see some pics!! and welcome to the forum!
Rob Amos
Happiness is a stock VW