We have purchased such substrates (4") from Esco Products in the past. They are
at 171 Oak Ridge Rd., Oak Ridge, NJ 07438-0155. Phone is (201)697-3700, fax is
(201)697-3011. Price (as of 2 years ago) was $45.00 for 4" 20-mil pyrex 7740
wafers with 60/40 finish.

Jeff Melzak
CWRU