Adorama and
B&H both have used camera sections. KEH is like the "Amazon of used camera gear." They have a huge stock, and they check the lenses well before selling. Unfortunately, I just checked and they don't have any right now. eBay doesn't have any currently, but they have 5 or 6 recently completed listings for a 600mm, ranging from $1,400 to $2,100.
However, no FD-EF adapter is "really good," since the flange distance necessitates optics in the adapter to allow focus to infinity. Once you start adding lens elements to your system, you're starting to chip away at image quality. You're better off using a Nikon F or Olympus OM adapter. Canon made a factory adapter right when they changed from EOS mount that's at least very high quality (no worse than using a teleconverter), but they currently sell for $1,000+ on the used market.