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.