James
A cheap newt secondary, especially a mass produced Dob type is unlikely to be 1/20 wave.
A google search for optical flats gives numerous suppliers : -
http://www.customscientific.com/flats.htm
http://opticalflats.com/ofpricelist.html
http://www.advancedoptics.com/optica...gle-sided.html
The optical flats need to be bigger than the cross section of the light cone they are reflecting to avoid vignetting.
You can always make your own. You need 3 x 4 inch blanks. I'd work in 10mm plate glass as it's already pretty flat to start with and clear both sides for testing.
Best of luck
Joe