Territorial Seeds has a vast collection of heirloom seeds. Yes, plants
from nurseries produce inferior
tasting tomatoes, like the various boys and girls. From Territorial, I
grow the following heirlooms: yellow pear, Costoluto (this one gets too
much disease though), Brandywine (the standard for
taste), san marzano (a plum type) and Stupice (a cold weather early
tomato). They have other ones that I am told rival Brandywine in
flavor. But when Brandywine is in season I need nothing else.

Obviously, there are things you can do to improve your tomatoes flavor,
regardless of type, in my case they make better tomatoes when given
lots of organic matter (a couple buckets per plant) and wood ash. And
the heirlooms crack and get diseases more often than the hybrids you
buy, so you have to be more careful with watering (not too much, not
too little, very constant), and you want to keep the plants well off
the ground, and the ground underneath well mulched, and no splashing
with the hose (I have drip).