If you want em cheap find someone wrecking a 1970 ish fiat 124 sport. These came with twin 40dco downdrought webers. I picked some up a few years back for $100.
Yep they would be the Weber 40 IDF's from the 1970-73 124 Sport BC (model code) 1600. The first model with the twin headlights and plain grille. Note that some of the early cars are in fact 1400's and have a single downdraft weber so this is not the one you would be after if a wrecker tells you he has a BC.
Also, some cars may be fitted with Solex equivalents.
Due to the rust bug any Euro/Fiat wrecker should have a ton of parts on their shelves.