I guess it all depends on location.... here in the Deep South, older bikes are still fairly common. In the Miami area where I often work, there are no older bikes available at all. I was in Iowa not long ago.... and picked up a fine old Schwinn for $25. I guess it varies from place to place.
BUT that being said.... I cannot see the lack of a Cantilever Frame preventing someone from building a COOL ride. There are ZILLIONS of old Schwinn Travellers, Pugeots, Raleighs and other old Road Bikes out there that make VERY FINE Rat-rods. My semi-Rat TREK is great fun to ride, and is just as COOL as my '53 Schwinn. Plus there are countless BMX and mountain bikes just rusting away. They ALL are suitable for Rat Rodding.
Rat-Rodding is NOT a "Cantilever ONLY" type of movement. It is FAR more creative than that. If you happen to find a Raleigh 3-speed.... then build a Rat-Raleigh! AND be proud of it! The whole idea is to take what you have, even if it looks like junk to the rest of the world, and build it into something COOL! Since the bike is YOUR creation.... YOU get to determine what is COOL and what is not.... and if others don't "get it" then it is THEY who are un-cool.
The concept of "COOL" in Rat-Rodding is not measured by what you STARTED with, but rather with what you CREATED FROM IT!
(DANG.... maybe I should have majored in Philosophy!)