I keep a look in backyards, I talk to people, I mention it at work, I search CL at least 3 times a day, and I also peruse local thrift stores regularly. That's how I got my best finds. So far, on CL, in backyards, and in thrift stores, I've found these beauties:
$50 Phillips Sports
$10 Raleigh Professional
$80 Univega Activa Trail
free Bridgestone Kabuki SuperSpeed
$10 Raleigh Rapide mixte
$30 Peugeot UO-8
$20 Fuji Palisade
Can't beat those prices for those bikes. Just keep an eye out, and don't buy crap.