Most Tbird parts houses get their interior seat covers and door panels from the same source. Kev gave ya a couple of leads. There are more in the Advertisements Forum below. Bob's Bird House, The Bird Nest, Tbird Hqs, MAC's, are some others. Call them all and ask them to send you a free catalog. Then you can compare costs. If your seat foams are shot, and falling apart under you, Jed Zimmerman, who is a member here, makes most of the seat foams for our Tbirds these days, and dealers buy from him. You will see his information in the Advertisements Forum also.
As for additives, it is good to add some zinc to your original engine. ZDDP is one that a lot of us use. Because of modern day catalytic converters, the zinc content in present day oil has been greatly decreased. Our engines don't have catalytic converters and do need plenty of zinc. If you do a Search on "zinc" you will get a lot of comments. Here is one, from simplyconnected.
"All oil with 40w in the numbers (like 15W-40) includes 1,000-ppm ZDDP. I like using Shell Rotella because it is a low ash oil. For those who don't know, when oil burns it forms an ash and your engine oil must deal with it."
As for your radiator, rather than buy a new one, unless it is just flat rusted out and shot, find a good radiator repair place and have yours repaired. That is what I did with mine. They re-cored it and it is like new again. You can get new radiators with more cores, aluminum, or exact replacements, etc., but you might be better off just getting yours restored.
And no, you are not breaking the rules for asking questions! That is what the Forums are for, to help each other fix and restore their Tbirds. And also, you are NOT ignorant! You just don't have the knowledge about these Tbirds that others do. One of these days, you might be the authority that others turn to because you have learned to fix your Tbird with your eyes closed! If you have not bought a '59 Shop Manual, you will want to. It will tell you a lot about how to fix things on your Tbird. There are other books about various parts of your Tbird that you might want to buy also, but a shop manual is a first must.
Let us know how we can help you and welcome again to Squarebirds.