If you're looking to build KITT yourself, the same vendors AZKNIGHT provided are all great. There are others who will build awesome parts, but they don't build the full turnkeys.
For fiberglass parts (in addition to those AZKNIGHT provided):
http://www.knightdesigns.com -- These parts are AMAZING. However, he is currently on hiatus and no telling on when he'll be back. But I have one of his lower consoles and you wouldn't believe how accurate and well-built it is.
http://www.knightpassions.net -- Probably the best bang for your buck. I have his gullwing and it is SOLID. He is in Europe but is able to ship overseas and is absolutely awesome to deal with.
For electronics:
http://www.knight-f2k4.com/ -- GET YOUR SCANNER HERE! The scanner on my car is one of his "older" ones and it still blows me away. Look on the replica boards for pictures of his dash electronics, they are awesome.
http://www.jupiterstore.com -- Also makes great electronics. Look for KittCARR and his recent pictures of his season 3/4 dash to see some of their work. I have their electronics in my upper console and love them.
For dash overlays:
http://sites.google.com/site/handeoverlays/home -- I haven't seen his work in person, but judging by the pictures I've seen on the replica boards, it's top-notch.
Sorry if I'm leaving anyone out. You can research the 2 forums that AZKNIGHT provided for the trusted vendors.
Now, whatever you do -
DO NOT BUY PARTS ON EBAY! (unless they specifically mention they were made by one of the trusted vendors). When buying from the trusted vendors, it will probably be more expensive and it could take a while to get your parts. But you could actually end up spending more in having these "cheaper" parts fixed. For example, you will notice that the eBay gullwing steering wheels do not come with the adapter already fastened to them. You need this adapter already on the wheel! Get the gullwing from ADIAT, Knight Passions, Knight Designs (when he’s making them again), Louisell, or Mark.
But I’d say join both boards, don’t be afraid to ask questions, and most important – have fun! Good luck!
-Brian (BDugan2145 on the replica forums)