Scosche and Metra have a lot of adapters.

Also check the website for Tune-Town - they do a good job of breaking things down per-car. I'd post some links for you, but I'm in a bit of a hurry to do some shopping... :)

Also, there's a great Miata website or mailing list that has a lot of audio information (I know because I kept getting hits for it while searching for other stuff - I'll see if I can find it later for you)

Bruno