Hi! I am in No. Georgia, so somewhat similar conditions.
Yes, septoria leaf spot. It sounds like you have done everything right and already know a lot about this. Yes, it is possible that they already had some of the fungus spores on them when you bought them. But I think septoria is pretty ubiquitous in rainy, humid climates. My tomato plants pretty much always have it by the end of the season.
Just keep pulling off affected leaves, especially the older ones, lower on the plant. You can spray the plant with some kind of fungicide. Organic fungicides include: hydrogen peroxide (straight out of the bottle which is a 3% solution), baking soda solution, diluted milk. Fungicides (all of them, including the bad chemical ones) work better preventively than as treatment for established disease, so the sooner you start the better. Only use one at a time, but you can rotate them, since it will need to be repeated every week or two. Be sure to spray the under sides of the leaves as well.
Unlike blight, IME tomato plants can keep surviving and producing with septoria for a long time as long as you keep up with it like this. By the end of the season they will be looking pretty ragged, with only leaves on top and bare stems below, but they will hang in!
Best wishes and let us know how it goes for you!
