SFGate

Bill Fry Construction - Wm. H. Fry Const. Co. Kohler does make a white undermount rectangular sink:
Kohler K-2297-G Kathryn 24" Undermount Bathroom Sink with Glazed Underside
http://www.faucetdirect.com/kohler-k-2297-g-kathryn-24-undermount-bathroom-sink-with-glazed-underside/p164508
so do a number of other manufacturers, such as Toto:
Toto LT156 Undermount Bathroom Sink
http://www.faucetdirect.com/toto-lt156-undermount-bathroom-sink/p558472
Best that you go to your local plumbing supply retailer and visit your sinks and faucets in person, because you'll be using them on a daily basis.
:) Rhoda

clksvg Thank you for your help. It is greatly appreciated!