Greg, try emailing or faxing your list to a few businesses you know are retailers of Corinthian & ask for a trade quote on the lot.
Local hardware, timberyards, etc.
Are they all flush doors, or fancy ones?
Dunno about the 420...Narrowest stock size I've seen is a 520. / hmmm.../ Might not be available, or if so, could be a custom made & pretty dear. Might be better getting another 820 & cut it down to width yourself (bit of mucking around tho' gluing the stile back in. Ensure you don't cut the lockblock side... All bets are off if its a fancy jobbie like a 4 panel )
Dunno also about 40mm thick. More likely either 35 or 45 in the solid core I think.