Just a crazy "gardening nut" idea here but do you live anywhere near his old home, Pam? Even if he hasn't been there for many years (I see Pam Sue was a cross from 1974) there might be plants still in his garden from when he lived there.
Begonias start very easily from cuttings . . . but do ask the present owners for cuttings if you see the one you're looking for.
Failing that, I just looked at the one online nursery I've ordered begonias from. Glasshouse Works has over 150 different begonias but they don't have 'Pam Sue'. But if you contact them, they may know how to track it down. Here's their web page, and there's a phone number and e-mail address at the bottom.
http://www.glasshouseworks.com...
