Gasoholics Anonymous

Searching eBay for "folding bicycles" turns up lots of options.

Foldabikes carries the Brompton bicycle. They also have travel pages extolling the possibility of using a folding bicycle for long distance travel.

West Marine carries a range of folding bicycles, because, well, that would be helpful for a boat owner. For similar reasons RvToyOutlet also has folding bicycles.

Downtube Bikes has a number of models, and has dealers in several places around the country.

Citizen Bikes has a number of models, but doesn't appear to have dealers.

Dahon is one of the principle makers of folding bicycles.

The Airframe folding bicycle available through the same dealer that carries the Brompton bicycle above.

Breezer Bikes has several folding bicycle designs, and they have an extensive dealer network around the country.

Bike Friday is a folding bike maker which has rave reviews on the Bike Access site (below). Be prepared for sticker shock.

Sportsbay carries a few folding bike models.

An extensive reource page on folding bicycles

Bike Access is an extensive information for traveling with bicycles.