Just always have to watch. there was a $25 Bridgestone that showed up on CL yesterday. Ad had been up a couple hours and I couldn't chase after it. Most of my CL bargains have been sheer luck and when the ad is vague to drop mails and ask the right questions.
Thrift stores like Goodwill and SA are hit and miss. Many of them if the bike is not in ridable condition they are throwing them in the trash One GW I walked around back and found 4 pallets of bikes stacked 15 or so deep each. The rideable bikes are often over priced box store bikes. I always look for the places that accept drop offs and I ask the people that take the drop off's if they have any bikes in the back. Found a few decent ones that way.

My best place has been a local thrift. I live in a better area and have a lot of 500k and up homes nearby. I hit that store every day on the way home from work and a couple times on Saturday. They got wise and have been raising the prices feeling the flippers out recently.