Because Thunderbird things must be licensed by Ford, there isn't much on the market
yet, which makes the Ford Collection catalog the best source right now.
Taxor is making a watch which has been reported about before.
www.taxor.com
Reyn Spooner will have a shirt available by February on their website. Some have made it to market already in some stores. Shirt is named "Tiki Birds" It has 1955 thru 2002(yellow) Thunderbirds on it, blue background with darker blue tiki figures.
A lot of non-clothing items have come across
ebay for the last 3 years - mostly made for or by Ford.
When there's a lack of specific clothing items, just go for colors that match or coordinate with your car. I'm in the process of accessorizing right now - jewelry - have to have something to do while I wait for my car.
You can also have shirts, jackets and hats customized at places that will do embroidery. They may already have some designs available or would be willing to come up with a design for you.