Is it too late to plant a tomato plant?

The two plants I put in pots do not look very good. They are growing and flowering and even have some fruit on them, but the leaves are yellowing with brown spots...not all leaves but enough of them to worry me.

I consulted someone in one of the stores where I purchased worm castings. He said some kind of master gardeners degree and that it sounded like a calcium/magnesium deficiency. So I bought some kind of liquid remedy but I am not convinced the plants will live and continue growing, eventually giving me a decent crop.

I was going to put another plant in a different pot and try again but I am just not sure if there is enough time left in the season to do it. If so, should I purchase a plant that already has a some green fruit on it? Or should I stay away from any bearing fruit (or flowers?) because it won't grow any more once transplanted? (Had that issue with a pepper plant last year.)

I would put the Mel's mix into the pot and grow it as I did not leave a spot for that in my box and also do not have a trellis built yet.

I have never had bad luck transplanting flowering/fruiting tomatoes but some people do. If I were you I would get at least one indeterminite cherry tomato like a sweet 100 and an early variety like an early girl. Those are easily found at greenhouses and should give you some decent fruit with time left in the season. And if they are in pots you could bring them in and out for a while when it starts to get cold at night at the end of the season!