These guys appear to know the differences between flathead models, but don't have all their stock on eBay. I'd contact them and tell them what you need.

Any GM 10Si alternator will work, but personally I'd find someone selling a quality rebuilt 1-wire.
http://www.ebay.com/itm/Flathead-Ford-Generator-Alternator-Conversion-Bracket-/250419408849?pt=Motors_Car_Truck_Parts_Accessories&vxp=mtr&hash=item3a4e28efd1

 

Hi Andy, since we live in the same house maybe you should come to me first.

Do an eBay search for "flathead alternator" you'll see them listed.

 

Seriously, Bob?!