I go to all brands of thrift stores: church, charity, for profit, and so on. Just google thrift store for your area/region, you will find a lot of them.

The thrift store search is hit and miss, mainly miss. >99% of the bikes I see at thrift stores are either kids bikes, or XMart bikes (usually both). And then, every so often, you will be rewarded. And you better grab it fast, because it will not be around long. There is no "good brand" thrift store, its really about the donations that come in. If all they get is Xmart bikes, then that is all they are going to sell.

It works better if you have other items you are looking for (and something more common). Otherwise, it can get pretty discouraging.

And I have done just as good this year finding deals on Craigs List and ebay (with a lot less looking).