Internet resources can be difficult to find, as most search engines (notoriously Google) will bring up quite a few blind links and irrelevant results. Here’s a list of sources that I have used and recommend:
Thunderbird Headquarters has a PDF version of their catalog available, though it can be difficult to find the exact part you need in that format. My local mechanic buys parts from this source with good success.
Bird Nest allows you to search and shop on-line; they also have a nice selection of used and rebuilt parts. I’ve bought many items here and have been happy with both the quality and service.
Dennis Carpenter also has decent prices and a good selection, but they are across the continent from me, which usually translates into slow shipping speeds.
I plan to replace my steering box soon, and when I do, it’ll be swapped out for one from Red-Head Steering Gears.
Forums and Clubs
Trying to match a paint color? PaintRef.com appears to have the tools you need.
A forum writer noted that interior paint can be purchased in spray cans to match the original from Automotive Touch-Up.
Shop manuals can be found at books4cars.com.
You can’t read forum posts too long before finding yourself sent to Jim Wulf’s site.
My own 64 (VIN 4Y83Z176987) is listed in the official Thunderbird Registry.