Here is where I have found them:
CTC-5 at an antique mall
CTC-11 at an indoor flea market
CTC-12 on ebay
CTC-15 on the curb
CTC-15 at the dump
Zenith 25MC30 in the newspaper classifieds
Zenith 25MC33 in a used furniture store
Admiral console at the dump
Admiral table model at the auction
Notice that there is no big "trend", I didn't find them all at one place. I don't have all of these sets anymore; these were found over a period of about 20 years.
Craigslist/Ebay makes the job a lot easier. The trouble with putting feelers out is that 99% of the people you talk will think you are describing a Sanyo from 1989, so you will get endless calls to drive an hour away only to find some dusty set from the 80s. There is no real foolproof way to describe them to somebody. Even a photo of one won't work. I've sent a picture to some thrift stores and still I get calls because somebody dropped off a 10-year old Toshiba. Don't fret, though. Just have patience and keep your eyes open.