Probably my best find was a banjolele from the 1930s that I got for next to nothing at a Salvation Army second hand store. A well built instrument in good shape, and with a hard case.
I also once found a really cheap Hammond spinet organ, with a built in Leslie speaker, that was cool. I had to get rid of the organ when I moved to a smaller apartment in a new town though.
There are often decent analog cameras at thrift stores and flea markets too, my favorites that I've gotten are a Pentax ME Super and a few nice lenses to go with it, an Olympus XA4, and a Meopta Flexaret II for medium format film.