Goodwill is what got me accidentally hooked on this stuff, but nearly all of my decent finds have come from fleabay as of late, including the European versions.
The fun part of thrifting is its complete unpredictability, which also happens to be its main drawback. The whims and generosity of the donors are already impossible to forecast as they are. On top of that, the store staff replenish the shelves and bins multiple times during the day, with no set pattern or logic, and there are many people out there fishing for these things, whose schedules are also beyond your control. With nary an exception, every single racquet that I had picked up from GW or SA over the past year and a half had been tagged on the very same day that I found it on the shelf, so I know for certain that anything remotely worth having is unlikely to sit there for more than a day; at least that's the case in my neck of the woods. This means that the only way to ensure that no rare gems would slip through my fingers would be to hit every store within my reach from morning to evening every day, seven days a week. This type of realization can easily produce the sort of paranoia and addictive urge that only medical intervention can control.
Yes, it costs a little more to fish on fleabay, but you save on time and buy more peace of mind with those few extra bucks, plus you have an infinitely larger body of water to locate your trophy in. Unless you happen to be immortal or are fortunate enough to have your own supply network, fleabay is probably your most realistic option if you are after something uncommon, which you wish to acquire before sanity and indifference catch up with you.