A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Date: 2017 Oct 3, 06:53 -0700
If you can tolerate the advertising, sciplus.com has cheap first surface mirrors:
Not so many now but there is a 70x194x3 mm for $3.95 and I’ve had good experiences with the company.
I think that someone posted that almost any newer glass is flat enough for AH purposes.