Perigord, I'm so glad that worked out for you.
Another website to try is Talbots.com, they have petite, plus and regular sizing.
What I personally found is that not all shirts fit the same from the same company/designer etc.
I bought Marina Rinaldi shirt and next time I couldn't buy as it didn't fit well.
Same with Lafayette and others, I think it's not only the company but the design of the shirt. If they change the model, the fit might change if that makes sense.
Lafayette has a website section dedicated to white shirts, but that doesn't mean they are fantastic.
Try landsend.com too. I love this thread.