Authentic Sour Dough??
Since moving to NYC 8 months ago, I have been very disappointed in the Sourdough that I have found in this city. I have been unable to find any authentic soft, sour tasting breads. Most of the Sourdoughs that I have found have taste like regular white breads.
It's difficult to find a real sour-sourdough even in San Francisco.
Sourdoughs are rustic breads, with hard crusts and a spongey airy crumb inside, so your best bet is to ignore anything billed as a "sourdough" and turn to the more rustic breads. What you'll lack in sour, you'll get back in a more satisfying texture, that will be much closer.
Sullivan Street Bakery and Amy's Bread both make large loaf shaped breads that will hit the spot. At Sullivan, it's the truccione saré which they compare to a sourdough, but they have another bread that they describe as having an "irregular crumb structure" that will do. I like the Italian Rustic at Amy's bread, and one other they have, for the same craving.
At the Union Square Greenmarket there's a vendor that sells a rustic sourdough that's pretty decent. It's the bread seller with the bakers rack, the pies, and the sourdough is usually available in bagged quarter sizes on top of their displays for the loose breads.
Again, these aren't soft outer crusts and sour insides, these are classic breads that will remind you of what used to be considered a sourdough bread. In SF, that would mean the bread at Tadich's (the real deal) or Tartine Bakery (which isnt't sour, but again, rustic).
As an alternative, the Portuguese breads available in the region are soft with a unique flavor. A great way to get away from the drab white bread problem you're talking about.
Union Square Greenmarket
Broadway and E 17th St, New York, NY 10003
Sullivan Street Bakery
533 W 47th St, New York, NY 10036
672 9th Ave, New York, NY 10036