Good urethane balls from the days of old? Hmmm, let me think...
As mentioned before, the urethane Hammers
AMF Black Angle
The Brunswick Rhinos (Black, Blue, Black Pearl, Red)
The Brunswick Rhino Pros (Black, Blue)
Columbia Black U-Dot
Columbia U2 (Black and Blue Pearl)
Columbia Beast Urethane
The Ebonite Turbos
Quantum Scarlet
Track Enforcer (the black urethane one, from the early 1990s)
I wish you luck in finding any of these, other than the Hammers on eBay.
As for newer releases, you may want to look at the Columbia Black Scout, Columbia Black Messenger, and Brunswick Blue Groove.
Hope this helps.