Where can I buy borosilicate (Pyrex) bakeware in the U.S.?
I believe the following companies advertise that they produce bakeware made out of borosilicate available in the US, :

Luminarc
Arcuisine Elegance

I can't speak to the boron/boron plus/zero boron debate, but those are the two I would investigate.

I know that Bodum also advertises borosilicate products, but I'm not sure if they make bakeware.