I haven't had any great thrift store finds, but my dream is to walk into a secondhand book shop one day and find William Henry Fox Talbot's Notebook L (never found to date).
It was 1999, and I had been looking for a copy of Bill Mauldin's book, Up Front, for many years. In a St. Vincent de Paul store in Ottawa I spotted one, minus the dust jacket, and I grabbed it! My wife, who was with me, told me afterwards that I didn't just grab it; she told me that I fairly leapt at the book shelf.
Does buying things from Goodwill via their website count? I've got quite a few great deals there lately including a gorgeous Minolta Hi-Matic E for $30 USD shipped including a nearly immaculate case and a perfect Yashica Electro 35 GSN for just over $25 USD shipped.
“You seek escape from pain. We seek the achievement of happiness. You exist for the sake of avoiding punishment. We exist for the sake of earning rewards. Threats will not make us function; fear is not our incentive. It is not death that we wish to avoid, but life that we wish to live.” - John Galt