Depends on what you're looking for. The Litespeed mentioned above is quite rare. What you'll find in garage sales and thrift stores is (a) junk and (b) quality bikes of the "bike boom" era of the 70s and into the 80s. Typically older parents finally selling the bike a kid left behind when the kid left for college or a non-rider finally realizing he's a non-rider.

Good chro-moly frames and solid Japanese components are the norm though every now and then you might find some Euro stuff. Much rarer though not impossible is the true gem.

A solid 80s bike weighing 24# or less can be had for $10-25 oftentimes. New rubber, cables and relube bearings and away you go.

The hunt can be most of the fun.