I've concentrated my "photo collecting" to 8th AF fighters, particularly the 4th FG, but I think Jack's (and Martin Kyburz, aka "Swiss Mustangs" - another prolific collector) collection came about basically the same way - US National Archives, eBay, trading with other collectors, copying or buying original collections ...
Jack's and Martin's collections differs from mine in that they have original prints in large part - I've been happy to acquire high-quality copies for my purposes (artwork reference). That has helped me in dealing with some collectors (like Jack and others) in that there's no "threat" to their originals. Jack, for one, sells copies on eBay.
One guy I know is the "Jack Cook" of 4th FG
original photos ... like Jack, he has had personal relationships with the original pilots
and ground crewmen (an untapped source with many WWII groups), and I helped him pay to scan and put a LOT of his collection on disc so I could have dupe discs. Win-win.
I can't speak for Jack or Martin, but if you're like them and concentrate your efforts on beating the bushes for photos for as long as they have, you'll have an amazing collection yourself!
Wade