Source of 30 pt. caliper library board

Does anyone know a source of 30 pt. caliper library board with an
alkaline reserve?   We have used this weight of board to make tuxedo
boxes for rare books and manuscript ledgers for several years, whenever
20 pt. board wouldn't provide adequate support.  We used to obtain it
from Process Materials (now Archivart, Div. of Heller & Usdan) and later
from Light Impressions.   Neither company now carries this product.
Minimum sheet size would need to be 28" x 36", grain preferably long but
short is okay, and color preferably light tan.  Substitute suggestions
would also be appreciated.  Thanks for your help.

Janet Riemer

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                  Conservation DistList Instance 4:14
               Distributed: Wednesday, September 19, 1990
                        Message Id: cdl-4-14-001
                                  ***