To save seed from a tomato just scoop out some seeds and ferment them for a few days. Then rinse clean in a strainer, dry them out, and put then away for next year. Some people just stick a few seeds to paper.
If it's planting time there are some fun options. You can plant a tomato slice for instance. You can also scoop the seeds out and plant them. They'll ferment right there in the potting soil and then sprout.
Locally grown organic tomatoes are probably a good place to start anywhere. Though one caveat, some growers have very nice greenhouses and season extension. They may grow some very long season varieties.
If you live in a nice long season area probably any tomato will work. Your profile says middle Georgia and that may fit the bill.
If the tomato is sold as a named variety you can look it up.
There is really no difference between saving seed from a purchased tomato and saving seed from one you grew.
You could also save seed from feral tomatoes you find, dumpster dived tomatoes, or any uncooked tomato you encounter- you could pick a fresh tomato seed out of a sandwich. I sometimes see feral tomatoes where someone probably stopped and ate their lunch.
If it is a hybrid think of saving it's seed as a breeding
project.
That said, it sure is nice to have nice well selected tomato varieties when I spend six months growing them. Do you have any tomato disease problems?