Plot out all the thrift stores in your city on a map, then go to the ones located in what you know to be 'rich' areas of town. You know the kind, crisp green lawns, old white people all over the place, fancy cars, etc. The thrift stores nearest those areas are the most likely to carry higher end bikes. Rich people buy nice bikes, and let them sit in the garage for 22 years, then take them to the nearest goodwill. Poor people buy crappy bikes, let them sit in the garage for 22 years, then take them to the nearest goodwill.

Then find out when they get new stock from the warehouse, and show up those days. Make it a date, just show up, browse around, and then leave empty handed, or with a nice bike! Some days I show up at my local thrifts, and there is nothing but a pile of Huffy 16" pixie bikes. But then you show up and there is a 10$ Raliegh Grand Prix Mixte in pristine blue looking up at you like a lost puppy who needs a home.

peace,
sam