Bethe - I make a trip once in a while to the DC area and buy Bainbridge 4-Ply Alpharag White - 8634 (mentioned earlier) in 32x40, 25 sheets, last time for about $200. That's only $2 for a 16x20. I used to use Westminster exclusively also.
I don't know if it's worth it to you, but it might be worth the trip. They are just off the beltway on the Fairfax side. You need to call ahead. Also, they will mail you a nice color sample box if you want. They have a number of whites.