Unfortunately, looking for a specific old bike is not very easy, but the trick is to look everywhere until you find it. If you ask this in Classic & Vintage, there may be people that will alert you to a bike, since many of them scour sites regularly looking for their own hard to find treasure.
Some ideas (some highly unlikely to be fruitful, but you never know).
The "For Sale section here (and other bicycle related forums)
Craigslist... look for people selling, or advertise what you are looking for.
Ebay.
Local bike club newsletters or meetings.
Bicycle Co-ops.
Local thrift stores.
Bike shops. They may carry used bikes, or know riders who have the bike you are looking for.
Garage sales.
Flea Markets.
Pay for a membership here and post in the "Want to Buy" section.
It is a little new for the section, but post in the ISO thread of Classic & Vintage, there are some lovers of old Bianchi bicycles there.
Pawn shops.
Keep an eye out for something celeste in any garage you pass, and ask if they want to sell...
Post on a college (or other) bulletin board.
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Slow Ride Cyclists of NEPA
People do not seem to realize that their opinion of the world is also a confession of character.
- Ralph Waldo Emerson