My preferred order for acquiring used bikes is:
#1 - bulky trash pickup
#2 - garage sales
#3 - second hand/thrift stores
#4 - pawn shops
In my home town we have what's called "bulky trash pick-up" twice a year - designed specifically for vegetation and big/bulky items that won't fit in the regular trash bins. You pile the stuff up according to what it is on the curb, and they have different trucks come by and pick up the various items. I download a copy of the schedule dates and neighborhoods from the city website, and drive around those neighborhoods the week before the city truck comes. Used to find ALOT of bikes there. The last year or so, it's dropped off considerably. People are fixing up their old bikes, and gas got real expensive last fall.
Garage sales help out the person directly selling it, thrift stores are one step removed from that. Pawn shops I try to stay away from as they get stolen merchandise and I hate to be supporting that kind of thing.
Also, let it be known that you fix up/rebuild bikes. I've gotten several bikes from people thru word of mouth or who knew I did bikes, as they get rid of them when the kids go off to college or cleaning out the garage/shed.